fix(decisioning): re-validate params through platform method's stricter subclass annotation

[bokelley]

Closes #596

Summary

_invoke_platform_method received an already-deserialized Pydantic instance typed as the library's base request (e.g. GetProductsRequest with extra="allow") and passed it directly to the platform method without consulting the platform method's own annotation. A DecisioningPlatform subclass that annotated its method with a stricter subclass (e.g. extra="forbid", custom @field_validator) had those rules silently bypassed at the dispatch boundary.

The fix adds _coerce_params_to_platform_type, called from _invoke_platform_method when arg_projector is None. It inspects the platform method's first non-self/non-ctx/non-context typed parameter annotation; when that annotation is a strict Pydantic subclass of the current params type, it re-validates via model_dump(mode="python") → model_validate(). A ValidationError from re-validation surfaces as INVALID_REQUEST / correctable (not INTERNAL_ERROR). The on_failure hook is called before re-raising so proposal-flow callers can release any consumption reservation taken before _invoke_platform_method.

Guard conditions that skip coercion: same type (no-op), unrelated annotation, VAR_POSITIONAL/VAR_KEYWORD parameters, get_type_hints() resolution failure (graceful degradation to dict pass-through).

What was tested

• pytest tests/test_decisioning_dispatch.py — 51 passed (6 pre-existing + 8 new coerce tests including: extra="forbid" rejection, same-type no-op, valid subclass passthrough, unrelated annotation no-op, param-name agnosticism, get_type_hints failure graceful degradation, on_failure hook fires on coercion failure, VAR_POSITIONAL guard)
• Full non-integration suite (excluding pre-existing network flake): 4153 passed, 0 failed
• mypy src/adcp/decisioning/dispatch.py --ignore-missing-imports — clean
• ruff check src/adcp/decisioning/dispatch.py — clean

Pre-PR review:

• code-reviewer: approved — 2 blockers found and fixed (on_failure not called before try block; VAR_POSITIONAL not guarded); nits surfaced in PR body; no remaining blockers
• dx-expert: approved — 1 blocker found and fixed (same on_failure/reservation-leak issue); doc paragraph for DecisioningPlatform subclass re-validation added to handler-authoring.md; nits: AGENTS.md callout, unrelated-annotation no-op docs note (acknowledged, not blocking)

Known nit (not fixed): The handler-authoring.md "Custom models too" paragraph could benefit from a code example showing the full DecisioningPlatform subclass pattern with extra="forbid" and @field_validator. Noted for a follow-up docs PR.

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See adcp#3121
for context.

Session: https://claude.ai/code/session_01GUtwfX2NfFEVUuZZBCA5qP

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