perf(server): lazy-load Pydantic schema generation to fix storyboard readiness flake

[bokelley]

Closes #412

Summary

_generate_pydantic_schemas(), _generate_pydantic_output_schemas(), and _apply_pydantic_schemas() previously ran at module import time, causing ~4.5s of Pydantic type-import and schema-generation work to race with the storyboard readiness probe. This produced "Agent unreachable" failures across PRs #391, #405, #406, #407.

Schema generation is now deferred to the first get_tools_for_handler() call, which fires during create_mcp_tools() at server construction — after the process is ready but before any wire request arrives. _PYDANTIC_SCHEMAS and _PYDANTIC_OUTPUT_SCHEMAS start as empty {} dicts and are populated via .update() so any external references (test files importing those symbols) stay valid. A _schemas_applied sentinel makes subsequent calls no-ops (~0ms on the hot path).

What tested

• pytest tests/test_mcp_schema_drift.py tests/test_tools_list_output_schema.py tests/test_spec_coverage.py tests/test_server_dx.py — 78 passed
• pytest tests/ (excluding integration/conformance) — 2816 passed, 17 skipped, 1 xfailed
• mypy src/adcp/ — no issues (749 source files)
• ruff check src/ — all checks passed
• Import-time delta measured: ~4.5s of schema generation moved out of import adcp.server onto first create_mcp_tools() call

Test infrastructure change

tests/conftest.py gains a scope="session" autouse fixture that calls _ensure_pydantic_schemas_applied() before any test body runs. This ensures tests that import _PYDANTIC_SCHEMAS or read ADCP_TOOL_DEFINITIONS["inputSchema"]/["outputSchema"] directly continue to see the Pydantic-generated schemas rather than the hand-crafted stubs.

Nits (not fixed — noted for awareness)

• _schemas_applied has no type annotation (mypy infers bool correctly from the global rebinding)
• No logger.warning if generation raises — exception propagates to create_mcp_tools() caller, same as old import-time crash behavior; per-tool failures still produce logger.debug as before

Pre-PR review

• code-reviewer: approved — call site in get_tools_for_handler is correct; schemas are applied before _register_handler_tools reads outputSchema from returned dicts; .update() mutation is safe; no blockers
• dx-expert: approved (1 blocker fixed — stale "at import time" in test_spec_coverage.py assertion message; fixed in follow-up commit); 3 nits noted above

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See adcp#3121
for context.

Session: https://claude.ai/code/session_01NnoQN3c6Wi5LY5DEUBp8W2

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