fix(spanner): improve grpc-gcp affinity cleanup and location-aware retries

[rahul2393]

Summary

This change updates grpc-gcp channel-affinity behavior and location-aware retry behavior in the Java Spanner client.

It does three things:

1. Uses random channel hints for grpc-gcp-backed single-use operations and multi-use transactions instead of reusing the session-affined hint.
2. Adds explicit affinity unbind handling for single-use read/query RPCs and terminal commit/rollback RPCs, including the retry-to-different-channel path.
3. Updates location-aware routing so retries for RESOURCE_EXHAUSTED do not go back to the same routed endpoint for the same logical request.

What Changed

grpc-gcp channel hints

When grpc-gcp is enabled:

• single-use read/query now uses a random channel hint
• single-use write (writeAtLeastOnce) now uses a random channel hint
• multi-use read-only transactions now get a random hint reused within that transaction
• multi-use read-write transactions now get a random hint reused within that transaction
• multiplexed single-use operations no longer use the bitset-based reserved single-use channel path

explicit affinity unbind

Added UNBIND_CHANNEL_HINT support and wired it through grpc-gcp call options.

This now:

• marks single-use read/query calls for unbind after completion
• preserves unbind when a streaming read/query is retried on a different grpc channel
• unbinds grpc-gcp affinity keys on commit
• unbinds grpc-gcp affinity keys on rollback

location-aware retry behavior

For location-aware routing:

• route selection now records tracing attributes/events
• if a routed endpoint returns RESOURCE_EXHAUSTED, the next retry attempt for that same logical request avoids that endpoint
• the skip is scoped by X-Goog-Spanner-Request-Id logical request identity, so unrelated transactions/requests do not nherit the exclusion
• if no alternate healthy replica is available, routing falls back to the default host

[gemini-code-assist]

Code Review

This pull request introduces a mechanism to unbind gRPC-GCP affinity keys after terminal RPCs (like Commit and Rollback) or when a specific channel hint is provided. It also adds logic to exclude endpoints that return RESOURCE_EXHAUSTED for the duration of a logical request's retry attempts. I have identified a discrepancy in the code comments within AbstractReadContext.java, where the comment claims the unbind flag is not carried on retries, despite the implementation explicitly adding it.