docs(agents): add typed-test checklist to the ESLint suppressions guidance

[easonLiangWorldedtech]

Closes #1419.

Adds a concise typed-test checklist to the ESLint Suppressions section of AGENTS.md so agents do not add blanket no-explicit-any suppression entries for new or substantially changed test files:

• Typed structural doubles: mock context/proxy classes typed against the real public types they stand in for, unknown for storage values and mock callback parameters.
• Bracket notation for private members; public members called without casts so their generic signatures keep checking arguments.
• A single documented construction-site double assertion instead of per-use as any casts.
• Required validation: check-types + narrowest Vitest suite + eslint --prune-suppressions with no new entry and no raised count.
• An exception requires a next-to-cast comment and maintainer approval.

Documentation-only change, grounded in the typing pass that retired the 137-count parallelMode suppression entry in #977 (f91e19c).

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