Add parallel reading support to S3SessionManager.list_messages()

[CrysisDeu]

Description

This PR adds parallel reading support to S3SessionManager.list_messages() to improve performance when retrieving multiple messages from S3. The implementation uses ThreadPoolExecutor to fetch messages concurrently, significantly reducing latency for sessions with many messages.

Key Changes:

• Added max_parallel_reads parameter to S3SessionManager.__init__() (defaults to 1 for backward compatibility)
• Implemented parallel S3 reads using ThreadPoolExecutor in list_messages()
• Support for both instance-level and per-call max_parallel_reads configuration
• Maintains message order regardless of completion order
• Handles individual message read failures gracefully without stopping the entire operation
• Added comprehensive test coverage including edge cases and mock-based verification

Backward Compatibility:

• Default max_parallel_reads=1 ensures sequential behavior (backward compatible)
• Existing code continues to work without modification
• No breaking changes to the API

Related Issues

#1164

Documentation PR

Type of Change

New feature

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Testing Details:

• Added 9 new test cases covering:
  • Default behavior (sequential reads)
  • Instance-level configuration
  • Per-call override functionality
  • Edge cases (few messages, many messages, pagination)
  • Mock-based verification of ThreadPoolExecutor configuration
• All existing tests continue to pass (1520 passed, 4 skipped)
• Verified message ordering is preserved
• Tested error handling for individual message read failures
• No new warnings introduced (existing warnings are pre-existing deprecation warnings)

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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