feat(spanner): support select for update

[arnabnandy7]

Problem

SpannerRepository.findById() uses Spanner's Read API, which cannot express a
FOR UPDATE clause. Custom SQL can lock the parent entity, but eagerly loaded
interleaved children are fetched through expression subqueries and are not
locked by an outer FOR UPDATE clause. Lazy interleaved child queries also did
not inherit locking options.

This can cause excessive transaction aborts when concurrent read-write
transactions read and update the same entity.

Fixes #3984.

Solution

This change adds opt-in FOR UPDATE support for Spring Data Spanner:

• Adds @Query(forUpdate = true) for derived and custom repository queries.
• Adds SpannerRepository.findByIdForUpdate(...).
• Adds SpannerQueryOptions.setForUpdate(true) for programmatic queries.
• Applies FOR UPDATE to generated parent queries and eager interleaved-child
  expression subqueries.
• Propagates locking to lazy interleaved-child queries.
• Rejects locking queries outside read-write transactions.
• Rejects combining forUpdate with DML queries.
• Documents the new APIs, transaction requirement, and contention trade-offs.

Testing

Added unit coverage for:

• Derived-query SQL generation.
• Eager interleaved-child locking.
• Query option behavior.
• findByIdForUpdate validation.
• Rejection outside read-write transactions.
• Existing query and repository behavior.

Targeted test result:

• 96 tests run.
• 0 failures.
• 0 errors.

Maven compilation and validation also complete successfully.