Wire checkout session customerEmail into Google Pay PM billing details

[cttsai-stripe]

go/o/req_BAWvlsMkOoCamA

Summary

• When confirming Google Pay in a checkout session, the backend validates payment_method.billing_details.email. Google Pay doesn't return an email by default (isEmailRequired is false in the checkout flow), so the PM gets created without one and confirm fails with checkout_email_missing.
• Reads customerEmail from the merged configuration (defaultBillingDetails.email, populated by CheckoutConfigurationMerger from the checkout session response) and passes it as a fallback into PaymentMethodCreateParams.createFromGooglePay().
• If Google Pay provides an email (from the card stored in the wallet), use it; otherwise fall back to the checkout session's customer email.

Changes

• PaymentMethodCreateParams.createFromGooglePay(): new billingEmailFallback param, used when Google Pay doesn't provide an email
• GooglePayPaymentMethodLauncherContractV2.Args: new billingEmailFallback field threaded through the launcher
• GooglePayConfirmationOption.Config: carries the fallback from the merged config
• ConfirmationOptionKtx: reads configuration.defaultBillingDetails?.email when constructing the GPay option
• TestGooglePay: new testCheckoutSessionWithFlowController test (requires real device)

Test plan

• testCheckoutSessionWithFlowController passes on real device with Google Pay (checkout session, FlowController, guest customer, automatic tax disabled)
• CI passes (detekt, apiCheck)

MOBILESDK-4640

🤖 Generated with Claude Code

[github-actions]

Diffuse output:

OLD: paymentsheet-example-release-master.apk (signature: V1, V2)
NEW: paymentsheet-example-release-pr.apk (signature: V1, V2)

          │            compressed            │           uncompressed           
          ├───────────┬───────────┬──────────┼───────────┬───────────┬──────────
 APK      │ old       │ new       │ diff     │ old       │ new       │ diff     
──────────┼───────────┼───────────┼──────────┼───────────┼───────────┼──────────
      dex │   4.6 MiB │   4.6 MiB │   +296 B │   9.7 MiB │   9.7 MiB │   +864 B 
     arsc │   3.8 MiB │   3.8 MiB │      0 B │   3.8 MiB │   3.8 MiB │      0 B 
 manifest │   5.8 KiB │   5.8 KiB │      0 B │  30.7 KiB │  30.7 KiB │      0 B 
      res │     1 MiB │     1 MiB │      0 B │   1.7 MiB │   1.7 MiB │      0 B 
   native │   2.7 MiB │   2.7 MiB │      0 B │   2.7 MiB │   2.7 MiB │      0 B 
    asset │  19.4 KiB │  20.9 KiB │ +1.5 KiB │  29.5 KiB │    31 KiB │ +1.5 KiB 
    other │ 245.5 KiB │ 245.5 KiB │    +18 B │ 505.4 KiB │ 505.4 KiB │      0 B 
──────────┼───────────┼───────────┼──────────┼───────────┼───────────┼──────────
    total │  12.3 MiB │  12.3 MiB │ +1.8 KiB │  18.4 MiB │  18.4 MiB │ +2.4 KiB 

 DEX     │ old   │ new   │ diff       
─────────┼───────┼───────┼────────────
   files │     1 │     1 │  0         
 strings │ 45332 │ 45333 │ +1 (+4 -3) 
   types │ 14262 │ 14262 │  0 (+0 -0) 
 classes │ 11604 │ 11604 │  0 (+0 -0) 
 methods │ 63336 │ 63335 │ -1 (+4 -5) 
  fields │ 41196 │ 41198 │ +2 (+2 -0) 

 ARSC    │ old  │ new  │ diff 
─────────┼──────┼──────┼──────
 configs │  320 │  320 │  0   
 entries │ 7492 │ 7492 │  0

APK

     compressed      │     uncompressed     │                                           
──────────┬──────────┼───────────┬──────────┤                                           
 size     │ diff     │ size      │ diff     │ path                                      
──────────┼──────────┼───────────┼──────────┼───────────────────────────────────────────
  8.9 KiB │ +1.5 KiB │   8.8 KiB │ +1.5 KiB │ ∆ assets/dexopt/baseline.prof             
  4.6 MiB │   +296 B │   9.7 MiB │   +864 B │ ∆ classes.dex                             
 58.9 KiB │    +16 B │ 130.8 KiB │      0 B │ ∆ META-INF/CERT.SF                        
    272 B │     +3 B │     120 B │      0 B │ ∆ META-INF/version-control-info.textproto 
  1.2 KiB │     -3 B │   1.2 KiB │      0 B │ ∆ META-INF/CERT.RSA                       
 55.6 KiB │     +2 B │ 130.7 KiB │      0 B │ ∆ META-INF/MANIFEST.MF                    
──────────┼──────────┼───────────┼──────────┼───────────────────────────────────────────
  4.7 MiB │ +1.8 KiB │    10 MiB │ +2.4 KiB │ (total)

DEX

STRINGS:

   old   │ new   │ diff       
  ───────┼───────┼────────────
   45332 │ 45333 │ +1 (+4 -3) 
  
  + , billingEmailOverride=
  + VLLJLLLLLZLLL
  + r8-map-id-5739503953e3285b9187ed55e9fd60693f2801efd3e683c799368e7cb31e2f0f
  + ~~R8{"backend":"dex","compilation-mode":"release","has-checksums":false,"min-api":23,"pg-map-id":"5739503953e3285b9187ed55e9fd60693f2801efd3e683c799368e7cb31e2f0f","r8-mode":"full","version":"8.13.19"}
  
  - VLLJLLLLLZLL
  - r8-map-id-66863c0c11bdc2fdf93d597bda75a207df52dc902a1ae5c6e1aa2cee305ce89f
  - ~~R8{"backend":"dex","compilation-mode":"release","has-checksums":false,"min-api":23,"pg-map-id":"66863c0c11bdc2fdf93d597bda75a207df52dc902a1ae5c6e1aa2cee305ce89f","r8-mode":"full","version":"8.13.19"}
  

METHODS:

   old   │ new   │ diff       
  ───────┼───────┼────────────
   63336 │ 63335 │ -1 (+4 -5) 
  
  + ai.i7 o(i7, b6, h8, j, int) → i7
  + oi.b <init>(p2, String, String, String, Long, String, s1, a, b, List, List, String)
  + sb.b b0(x, a, j0, b, List, String) → n
  + ug.k0 <init>(b0, String, long, String, String, a, b, q, boolean, String, List, String)
  
  - ai.i7 o(i7, h8, j, int) → i7
  - androidx.lifecycle.u0 D(Object) → Object
  - oi.b <init>(p2, String, String, String, Long, String, s1, a, b, List, List)
  - sb.b b0(x, a, j0, b, List) → n
  - ug.k0 <init>(b0, String, long, String, String, a, b, q, boolean, String, List)
  

FIELDS:

   old   │ new   │ diff       
  ───────┼───────┼────────────
   41196 │ 41198 │ +2 (+2 -0) 
  
  + oi.b p: String
  + ug.k0 p: String

[amk-stripe]

There's also a customerEmail on the checkout session object -- is that always the same as this email? Or should we additionally fallback to that customerEmail value too?

[porter-stripe]

+1, we need to ensure that if customerEmail is on the CS we try to use it if available. I'm guessing we merge it into the defaultBillingDetails tho?

[cttsai-stripe]

Right. I added tests to cover that.