refactor(gateway): deepen ACP event normalization into event_encoder module

[DIodide]

Applies the improve-codebase-architecture skill's top deepening recommendation (chosen from 30 surfaced candidates).

What

The pure ACP-update→SSE normalization — the highest-churn encoding surface (it shifts with every agent/model update) — was buried in the 3179-line session_manager.py god module, interleaved with the stateful session lifecycle, relay, permission, and provisioning machinery. This moves the stateless dict→dict encoding rules into a new sibling module app/services/agents/event_encoder.py behind a small interface:

• normalize_session_update, normalize_sdk_task_message, parse_sdk_compaction, parse_elicitation_fields (+ the SDK_TASK_MESSAGE_FILTERS list and COMPACTION_SUMMARY_PREAMBLE marker the session meta/replay builders still reference).
• The 8 leading-underscore helpers + regex/phase constants become private internals of the module (internal seam, not part of the interface).

session_manager.py shrinks 3179 → 2595 lines and imports the public entry points.

Why

• locality: all update-to-SSE encoding rules concentrate in one named module instead of scattered through a 3k-line file.
• leverage: the encoding rules are reusable by any transport without touching session state.
• navigability: a maintainer reasons about encoding without scrolling the god module.
• The deletion test passes: deleting event_encoder would scatter the rules back across the session module.

Pure relocation — behaviour-preserving

No callsite logic changed. Per "the interface is the test surface" / "replace, don't layer", the existing tests (test_agent_normalize, test_agent_questions, test_agent_usage_gateway) re-point to the real seam (event_encoder) rather than session_manager re-exporting.

Verified: a 5-agent adversarial pass confirmed the moved code is byte-identical (diff exit 0), no circular import, callsites unchanged, no test weakened — and an xhigh code-review returned 0 findings. fastapi 362 passed, ruff clean.

No user-facing change → no CHANGELOG entry.