refactor(spanner): decompose AbstractReadContext and deduplicate stream creation

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`.

Author: olavloite