docs(runtime): clarify filtered session history retention

[baba9811]

Clarifies that Python RunConfig.get_session_config limits the loaded event view while preserving stored history when new events are appended. This accompanies the Redis filtered-history preservation fix in google/adk-python.

Testing

• mkdocs build --strict passes and indexes 235 pages. It reports an existing missing migration-page anchor at INFO level; this change adds no links.
• mkdocs serve returns HTTP 200 and renders the clarification as part of the existing list item.
• Verified the behavior with direct appends and Runner turns on Redis, memory, SQLite and SQLAlchemy SQLite.

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[joefernandez]

Technical review report

• Review Score: 100
• Overall assessment: 🟢 Low Risk
• Summary reasoning: The three added sentences in docs/runtime/runconfig.md state that GetSessionConfig filters (num_recent_events, after_timestamp) only narrow the loaded view and that new events append to the full stored history. I verified this against every session service shipped in the current release, adk-python v2.9.1, and the claim is correct in each. The API referenced is released, not merged-but-unreleased.
• Recommended action: No concerns, approve. The change adds accurate clarification to an already-correct bullet, touches no code samples, and introduces no new API surface. The only observation is stylistic (mild redundancy between the two new sentences), not a technical risk.
• Number of lines changed: XS, 4
• Number of files changed: 1

Verification detail

The changed text is at docs/runtime/runconfig.md:76-80, inside a section tagged "Supported in ADK: Python", which matches the fact that get_session_config exists only on Python's RunConfig: run_config.py:262-280.

Claim 1 — "These filters limit the loaded view without deleting stored events":

• InMemorySessionService slices a copy of the session (_copy_session, then copied_session.events = copied_session.events[-num_recent_events:]); the stored session object is untouched: in_memory_session_service.py:202-217.
• DatabaseSessionService applies the filters as a WHERE timestamp >= ... / LIMIT on the read query only: database_session_service.py:706-734.
• SqliteSessionService likewise, as SQL AND timestamp >= ? / LIMIT ?: sqlite_session_service.py:309-326.
• VertexAiSessionService passes after_timestamp as a server-side list filter and trims num_recent_events client-side on the returned list: vertex_ai_session_service.py:240-300.

Claim 2 — "New events are appended to the stored history, preserving older events excluded from the loaded view":

• In-memory: append_event appends to the canonical storage_session, even when the caller holds a filtered copy: in_memory_session_service.py:354.
• Database: append_event inserts a new StorageEvent row and never rewrites the event table: database_session_service.py:976.
• The Runner passes run_config.get_session_config straight into session_service.get_session() on the load path, so filtering is confined to the read: runners.py:895-900.

No risk items found. One optional, non-scoring wording note: "without deleting stored events" and "preserving older events excluded from the loaded view" say the same thing twice; the bullet could be tightened to a single sentence, e.g. These filters limit the loaded view only: stored events are not deleted, and new events are appended to the full stored history.

[joefernandez]

Thanks for the update!