refactor(spanner): decompose AbstractReadContext and deduplicate stream creation

[olavloite]

This is a pure refactoring with zero behavioral or public API changes:

1. Subclass Extraction:
   • Extract SingleReadContext, SingleUseReadOnlyTransaction, and MultiUseReadOnlyTransaction from inner classes in AbstractReadContext into dedicated package-private top-level classes in com.google.cloud.spanner.
   • Add class-level Javadocs describing the lifecycle and purpose of each read-only transaction type.
2. Stream Creation Deduplication:
   • Deduplicate shared ResumableStreamIterator construction, stream listener registration, transaction selector setup, and RPC execution error handling between executeQueryInternalWithOptions and readInternalWithOptions in AbstractReadContext using private helper methods (createStreamingResultSet, executeCall, prepareTransactionSelector).
3. Import & Reference Cleanup:
   • Remove obsolete inner-class imports across SessionImpl and BatchClientImpl.
   • Remove redundant superclass static imports in the extracted classes.
   • Update AsyncReadOnlyTransactionTest to reference the top-level MultiUseReadOnlyTransaction.

[gemini-code-assist]

Code Review

This pull request refactors AbstractReadContext.java by extracting several inner classes (SingleReadContext, SingleUseReadOnlyTransaction, and MultiUseReadOnlyTransaction) into separate top-level files, and consolidates streaming result set creation using a new createStreamingResultSet helper method. Feedback on these changes highlights a behavioral regression where the override of prepareIteratorForRetryOnDifferentGrpcChannel() was omitted in the refactored stream creation, which prevents channel rotation on retry. Additionally, it is recommended to avoid using the fully qualified class name for ExecuteSqlRequest.QueryMode since the class is already imported.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request refactors AbstractReadContext by extracting several inner classes (SingleReadContext, SingleUseReadOnlyTransaction, and MultiUseReadOnlyTransaction) into their own top-level files. Additionally, it introduces a helper method createStreamingResultSet to consolidate streaming result set creation logic. A review comment identifies a regression in this helper method where the override for prepareIteratorForRetryOnDifferentGrpcChannel() was omitted, which prevents channel affinity retries for multiplexed sessions; a suggestion is provided to restore this override.