feat(spanner): export SpannerRetryPolicy

[fornwall]

The Spanner client installs SpannerRetryPolicy as the default retry policy for RPCs that do not configure their own, but the type was pub(crate).

Downstream users who wanted to bound the client's default behavior (e.g. cap attempts or elapsed time with RetryPolicyExt::with_attempt_limit / with_time_limit) while keeping the Spanner-specific error classification - retrying transport/IO errors on idempotent requests on top of the strict AIP-194 checks - had to maintain a byte-for-byte behavioral copy of the policy, which can silently drift when the crate changes.

Make the retry_policy module and SpannerRetryPolicy (+ its new constructor) public, with rustdoc describing the policy's behavior and an example decorating it with RetryPolicyExt. No behavior changes.

Precedent in other language clients

The other official Spanner clients treat the default retry configuration as public, user-tunable API rather than an internal detail; this PR brings the Rust preview client in line.

• Go (cloud.google.com/go/spanner): the Spanner-specific default backoff is an exported package variable, spanner.DefaultRetryBackoff, and the handwritten client's ClientConfig.CallOptions *vkit.CallOptions exists precisely "for providing custom retry settings that override the default values". The generated apiv1.Client additionally exposes its CallOptions field publicly, pre-populated with the per-RPC default retryers (spanner/apiv1/spanner_client.go), so callers can inspect and rebound the defaults directly.
• Java (googleapis/java-spanner): SpannerOptions.Builder.getSpannerStubSettingsBuilder() is public and documented for setting custom RetrySettings per gRPC method (including the documented applyToAllUnaryMethods idiom); the builder starts from the public generated SpannerStubSettings defaults, so the default per-method RetrySettings are obtainable via public API (e.g. executeSqlSettings().getRetrySettings()) and can be modified with toBuilder() rather than re-implemented.
• Python (googleapis/python-spanner): every generated and handwritten data method takes a public retry= parameter accepting a google.api_core.retry.Retry (e.g. Snapshot.execute_sql), and the handwritten layer exports a module-level default, DEFAULT_RETRY_BACKOFF = Retry(initial=0.02, maximum=32, multiplier=1.3) in spanner_v1/database.py. One nuance: the per-RPC generated defaults live as default_retry values inside the generated transport (transports/base.py) — visible source but not exported constants — though any of them can be replaced per call.
• Node.js (googleapis/nodejs-spanner): the default per-method retry codes and backoff parameters ship as data in the published package (src/v1/spanner_client_config.json), and the generated client's constructor accepts a documented clientConfig override merged over those defaults via gax constructSettings, plus per-call gax CallOptions retry settings (src/v1/spanner_client.ts). The default here is packaged data rather than an exported symbol, but it is still user-visible and overridable without re-implementation.

[gemini-code-assist]

Code Review

This pull request makes the SpannerRetryPolicy and its module public, and adds detailed documentation along with a usage example. The feedback highlights a critical issue where linking to the privately imported Aip194Strict in public documentation will trigger a rustdoc warning and fail the CI build. Fully qualifying the link to google_cloud_gax::retry_policy::Aip194Strict is recommended to resolve this.

[olavloite]

/gcbrun

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.95%. Comparing base (6a88ca4) to head (cbdc970).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6048   +/-   ##
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  Coverage   96.95%   96.95%           
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  Files         248      248           
  Lines       62379    62379           
=======================================
+ Hits        60480    60482    +2     
+ Misses       1899     1897    -2     

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[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request exposes the retry_policy module and the SpannerRetryPolicy struct publicly, including its new constructor, and adds comprehensive documentation with usage examples. The feedback points out an unresolved rustdoc reference link in the newly added documentation that should be updated to standard inline link syntax to avoid warnings during documentation builds.

[fornwall]

@olavloite Thanks for the review!

If you have the time, please review #6037 as well - it's a major performance hit with many rows, with a simple and safe fix.