Correct documented chat message types

[Chris0Jeky]

Summary

• Correct the chat API example to use the implemented proposal-reference message type.
• Document all seven values from ChatMessage.ValidMessageTypes, including when each is emitted.

Verification

• node scripts/check-docs-governance.mjs
• git diff --check
• Backend, frontend, and Playwright checks not run; this is a documentation-only change.

Documentation

• docs/STATUS.md updated (not applicable; no shipped behavior changed)
• docs/IMPLEMENTATION_MASTERPLAN.md updated (not applicable)
• docs/TESTING_GUIDE.md / docs/MANUAL_TEST_CHECKLIST.md updated (not applicable)

Tracking

• Closes docs/api/CHAT.md documents a message type that does not exist and omits four that do #2078
• Linked issue project item status reviewed (Review while this PR is open)

CI Workflow Validation

• Not applicable; no workflow, deploy, script, or project-file changes.

Risk Notes

• Security impact: None.
• Behavior/regression risk: None; documentation only.
• Follow-up tasks: None.

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[Chris0Jeky]

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[Chris0Jeky]

Review gate (Codex credits exhausted, SC-9): one fresh-context reviewer verified every changed line against the domain whitelist in ChatMessage.cs and the ChatService emission sites; the corrected example matches ChatApiTests. Verdict SHIP. Two LOW doc-truth residuals outside the fix (status semantics wording, DATA_MODEL.md omits parse-hint) tracked in #2488. Merging.