Python: forward Foundry context instructions

[he-yufeng]

Summary

• forward system/developer messages from Foundry context providers as request instructions
• keep agent-level instructions and skills/context instructions together instead of dropping the context side
• add a regression test for SkillsProvider-style context instructions

Fixes #5883.

To verify

• uv run pytest packages\foundry\tests\foundry\test_foundry_agent.py::test_raw_foundry_agent_chat_client_prepare_options_forwards_context_instructions packages\foundry\tests\foundry\test_foundry_agent.py::test_raw_foundry_agent_chat_client_prepare_options_accepts_function_tools packages\foundry\tests\foundry\test_foundry_agent.py::test_raw_foundry_agent_chat_client_prepare_options_strips_client_side_fields -q --basetemp .tmp\pytest -p no:cacheprovider
• uv run ruff check packages\foundry\agent_framework_foundry\_agent.py packages\foundry\tests\foundry\test_foundry_agent.py
• uv run ruff format --check packages\foundry\agent_framework_foundry\_agent.py packages\foundry\tests\foundry\test_foundry_agent.py
• uv run pyright packages\foundry\agent_framework_foundry\_agent.py packages\foundry\tests\foundry\test_foundry_agent.py
• uv run python -m py_compile packages\foundry\agent_framework_foundry\_agent.py packages\foundry\tests\foundry\test_foundry_agent.py
• git diff --check

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[he-yufeng]

Synced this branch with the latest upstream/main and re-ran the targeted checks after the merge.

Verified locally:

uv run --no-sync pytest packages/foundry/tests/foundry/test_foundry_agent.py -q
uv run --no-sync ruff check packages/foundry/agent_framework_foundry/_agent.py packages/foundry/tests/foundry/test_foundry_agent.py
uv run --no-sync ruff format --check packages/foundry/agent_framework_foundry/_agent.py packages/foundry/tests/foundry/test_foundry_agent.py
uv run --no-sync python -m py_compile packages/foundry/agent_framework_foundry/_agent.py packages/foundry/tests/foundry/test_foundry_agent.py
uv run --no-sync pyright packages/foundry/agent_framework_foundry/_agent.py packages/foundry/tests/foundry/test_foundry_agent.py
git diff --check upstream/main...HEAD

All passed.