Check metadata length for bolt11 invoices

[elnosh]

I mentioned this here

Similar to the description check, I think this length should be checked that is less than the limit for tagged fields in bolt11 invoices. Otherwise it can hit this assert during serialization.

[ldk-reviews-bot]

I've assigned @valentinewallace as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[ldk-claude-review-bot]

No issues found.

The change correctly checks payment metadata length against MAX_TAGGED_FIELD_DATA_BYTES (639). I verified the threshold is right: a 640-byte metadata yields ceil(640*8/5) = 1024 base32 groups, which would trip the len < 1024 assert at ser.rs:427, while 639 bytes yields exactly 1023 groups. The error-recording pattern (self.error = Some(...) checked later in build_raw at lib.rs:734) is consistent with how description() handles DescriptionTooLong, and the test covers the 640-byte rejection case.

Minor (non-blocking) observations, not posted inline as they are style preferences:

• The new CreationError::PaymentMetadataTooLong doc comment hardcodes "639 bytes" while Description::new's doc was updated to reference the constant — slightly inconsistent.
• No test asserts the boundary (639 bytes accepted), only the over-limit (640) rejection.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.89%. Comparing base (1c1a4ad) to head (87ea15c).

Files with missing lines	Patch %	Lines
lightning-invoice/src/lib.rs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4740      +/-   ##
==========================================
- Coverage   86.90%   86.89%   -0.01%     
==========================================
  Files         161      161              
  Lines      111680   111688       +8     
  Branches   111680   111688       +8     
==========================================
+ Hits        97052    97054       +2     
- Misses      12115    12126      +11     
+ Partials     2513     2508       -5     

Flag	Coverage Δ
fuzzing-fake-hashes	7.77% <0.00%> (-0.01%)	⬇️
fuzzing-real-hashes	29.82% <0.00%> (-0.01%)	⬇️
tests	86.24% <90.00%> (-0.01%)	⬇️

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