fix(a2a): forward get_session_config in _prepare_session()

[vnykdfd]

Link to Issue

Closes: #6410

Problem Description

Environment

• ADK Version: 2.4.0 (latest main branch)
• Python: 3.10-3.14 (tested on 3.13)
• Component: A2A executor (legacy path)

Bug Summary

_prepare_session() in A2aAgentExecutor ignores GetSessionConfig(num_recent_events=N) by not forwarding it to session_service.get_session(), causing the entire event history to be loaded instead of respecting the configured limit.

Reproduction

from google.adk.agents.run_config import RunConfig
from google.adk.sessions.base_session_service import GetSessionConfig

# Configure to load only 10 recent events
run_config = RunConfig(
    get_session_config=GetSessionConfig(num_recent_events=10)
)

# Bug: _prepare_session() calls get_session() WITHOUT config parameter
# Result: All events loaded (e.g., 291 events instead of 10)

Expected Behavior

Session service should load at most 10 events when num_recent_events=10 is configured.

Observed Behavior

Session service loads ALL events regardless of get_session_config setting.

Root Cause

_prepare_session() (line 320-324) calls session_service.get_session() without passing config= parameter:

# BEFORE (missing config parameter)
session = await runner.session_service.get_session(
    app_name=runner.app_name,
    user_id=user_id,
    session_id=session_id,
    # ← Missing: config=get_session_config
)

Impact

For sessions with hundreds of events:

• 10-100x more database I/O than configured
• 200-300ms added latency per request (observed in production)
• Higher memory usage (~300KB vs ~10KB)
• Higher token costs (sending excessive context to LLM)

Solution

Extract get_session_config from run_request.run_config and forward it to session_service.get_session():

# AFTER (with fix)
get_session_config = None
if run_request.run_config:
    get_session_config = run_request.run_config.get_session_config

session = await runner.session_service.get_session(
    app_name=runner.app_name,
    user_id=user_id,
    session_id=session_id,
    config=get_session_config,  # ← ADDED
)

Testing Plan

Unit Tests Added

• test_prepare_session_passes_get_session_config - Verifies config is forwarded correctly
• test_prepare_session_none_run_config - Handles run_config=None gracefully
• test_prepare_session_no_get_session_config - Handles missing get_session_config

Test Results

Local pytest (Python 3.13):

$ pytest tests/unittests/a2a/executor/test_a2a_agent_executor.py -v
==================== 29 passed, 1 skipped in 1.50s ====================

Full A2A test suite:

$ pytest tests/unittests/a2a/ -v
==================== 372 passed, 17 skipped in 2.54s ====================

Pre-commit hooks:

$ pre-commit run --all-files
All checks passed ✅

Manual E2E Verification

Reproduction script confirms the fix works (script available on request).

Changes Made

Files Modified

1. src/google/adk/a2a/executor/a2a_agent_executor.py (+4 lines)

   • Extract get_session_config from run request
   • Pass it to session_service.get_session(config=...)

2. tests/unittests/a2a/executor/test_a2a_agent_executor.py (+103 insertions, -14 deletions)

   • Added 3 new unit tests
   • Fixed existing tests: replaced Mock(spec=RunConfig) with RunConfig() instances
     (Pydantic v2 fields not accessible via class-level Mock spec)

Backward Compatibility

✅ Fully backward compatible - no breaking changes
✅ Existing behavior unchanged when get_session_config is None
✅ All 372 A2A tests pass

Checklist

• Read CONTRIBUTING.md
• Self-review performed
• Added code comments for clarity
• Added unit tests proving fix works
• Tests pass locally
• Pre-commit hooks pass
• License headers present on all files
• No dependent changes required
• CLA signed

Additional Context

This bug only affects the legacy A2aAgentExecutor path. The newer _A2aAgentExecutor implementation already handles session loading more efficiently by passing num_recent_events=0 during existence checks.

Tested locally on Python 3.13. CI will verify compatibility with Python 3.10-3.14.