Add session-pinned Ollama base URL balancing

[AminMahpour]

Summary

• add Ollama base_urls pool support with load_balance = "session_round_robin"
• pin each Chainlit session to one Ollama URL while keeping the base URL override behavior intact
• update docs and examples to show the new model config shape
• add regression tests for config parsing, session assignment, and MCP session cache behavior

Testing

• uv run pytest tests/test_deepagent_runtime_rag.py::test_runtime_config_reads_ollama_base_urls_from_toml tests/test_deepagent_runtime_rag.py::test_agent_runtime_assigns_ollama_base_urls_per_session tests/test_deepagent_runtime_rag.py::test_stateful_mcp_reuses_session_per_chainlit_session -q
• uv run pytest -q

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve explicit base_url when base_urls is configured

When base_urls is present, this branch overwrites base_url with base_urls[0], so an explicitly configured [model].base_url is ignored. That breaks the documented/expected split where base_url remains the default (including RAG embedding endpoint) while base_urls is only the chat pool. A config like base_url=http://hostA and base_urls=[http://hostB,http://hostC] will silently route default and embedding traffic to hostB instead of hostA.

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[chatgpt-codex-connector]

Evict stale model URL assignments after session turnover

Each new session/thread key is stored in _model_base_url_assignments and never removed, so a long-lived runtime accumulates one entry per historical session. Since chat/session IDs are effectively unbounded over time, this creates unbounded in-memory growth and stale pinning state even after sessions are closed.

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