[Cosmos][Spec] Adopt cross-SDK Hedging Detection API on DiagnosticsContext

[NaluTripician]

Draft. This PR adopts the cross-SDK Hedging Detection API for the Rust SDK as a spec-only change. Refs #4410.

Targets release/azure_data_cosmos-previews (same as #4330). Doc-only — exactly one file changed.

What this PR does

Adds sdk/cosmos/azure_data_cosmos/docs/HEDGING_DETECTION_API_SPEC.md, which specifies three additive accessors on DiagnosticsContext (CosmosDiagnosticsContext re-export):

• hedging_started() -> bool
• requested_regions() -> Vec<&Region> (dispatch order, dups allowed)
• responded_regions() -> Vec<&Region> (completion order, dups allowed)

plus the Retry → OperationRetry rename mechanics, the public re-export decision, and a reconciliation table against #4330's HEDGING_SPEC.md §10.

What this PR does NOT do

No production code. No tests. No CHANGELOG (drafted in the spec; landed by the future implementation PR).

Sequencing

1. PR Cosmos: Adds Cross-Region Hedging Design Spec to Driver Crate #4330 merges first. Brings HEDGING_SPEC.md and the hedge_diagnostics: Option<HedgeDiagnostics> design contract onto release/azure_data_cosmos-previews.
2. This spec PR merges.
3. Future hedging implementation PR (populates HedgeDiagnostics, emits ExecutionContext::Hedging).
4. Future Hedging Detection API implementation PR (adds the three accessors, performs the Retry → OperationRetry rename, lands the CHANGELOG entries drafted here).

Cross-references

• Tracking issue: [Cosmos] Adopt cross-SDK Hedging Detection API on DiagnosticsContext — spec only (depends on #4330) #4410
• Hedging Design Spec PR: Cosmos: Adds Cross-Region Hedging Design Spec to Driver Crate #4330 (head SHA 3647b404e5366e1e91344ffa5a5929a7a6845c38 at the time this draft opens — cited in the spec's reconciliation table per SE-023 mitigation step (a)).
• Cross-SDK design doc: "Hedging Detection API" (.NET / Java / Python / Rust).

Cross-SDK gate decisions reflected here

ID	Decision	Reflected in spec
Q4	Sibling spec file (not §10 amendment to #4330)	File location
Q5	Wire-format break on ExecutionContext ACCEPTED. New variant serializes as "operation_retry"; #[serde(alias = "retry")] preserves back-deserialization; CHANGELOG entries on both crates explicit. No #[serde(rename = "retry")].	§4 + §9
Q6	Re-export Option B recommended (project to primitives; ExecutionContext stays driver-private)	§5

Reviewers

cc @kundadebdatta @analogrelay @FabianMeiswinkel @simorenoh @NaluTripician

CODEOWNERS will auto-add @Azure/azure-cosmos-rust-sdk.

Refutations of cross-SDK design doc §10 (carried in spec §2 / §4.2)

1. ExecutionContext::Hedging is not dispatched today (only Initial, Retry, TransportRetry, RegionFailover are emitted).
2. ExecutionContext and RequestDiagnostics are not re-exported from azure_data_cosmos.
3. The Retry → OperationRetry rename cannot be a #[deprecated] re-export — Rust has no enum-variant aliases. The correct mechanism is #[non_exhaustive] + new variant + #[deprecated] on the old variant + dispatch-site updates + #[serde(alias = "retry")] on the new variant.
4. The cross-SDK accessors and Cosmos: Adds Cross-Region Hedging Design Spec to Driver Crate #4330's HedgeDiagnostics field coexist; reconciliation table in spec §6.