fix(docs): use serve_window_sec in TMP Identity Match overview example

[brandonling27]

Summary

• The overview walkthrough's identity_match_response example used the legacy ttl_sec field, which fails schema validation — the schema requires serve_window_sec.
• The two fields are not synonyms: serve_window_sec is a per-package single-shot fcap (the publisher must re-query Identity Match after serving once per eligible package), not a router response cache TTL. Updated the example's accompanying explanation to reflect this.
• Updated the matching fixture in tests/example-validation-simple.test.cjs so the TMP Identity Match response — web (overview walkthrough) case validates against the schema.

Note: the same ttl_sec vs. serve_window_sec drift exists in the surface walkthroughs (surfaces/web.mdx, surfaces/mobile.mdx, surfaces/ctv.mdx, surfaces/retail-media.mdx, surfaces/ai-assistants.mdx) and in context-and-identity.mdx. Those aren't covered by the current validation test and are left for a follow-up to keep this PR scoped to the failing test.

Test plan

• node tests/example-validation-simple.test.cjs — 36/36 passing; the previously failing TMP Identity Match response — web (overview walkthrough) case now passes.
• Mintlify renders docs/trusted-match/index.mdx Step 4 cleanly with the new explanation.

🤖 Generated with Claude Code

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