REST API: Conditional post updates (If-Unmodified-Since) and Last-Modified headers

[pranav-inamdar]

Summary

Adds support for conditional POST/PUT semantics when updating posts via the REST API:

• If-Unmodified-Since: If the client sends this header and the post’s GMT modified time is later than the parsed header date, the update is rejected with 412 Precondition Failed and error code rest_precondition_failed, with no changes applied.
• Last-Modified: Successful GET and PUT responses for posts (including the attachment path in update_item) include a Last-Modified header (IMF-fixdate, GMT) derived from post_modified_gmt, so clients can store and replay that value as If-Unmodified-Since on later writes.

Invalid or unparseable If-Unmodified-Since values are ignored so a bad header does not block edits.

PHPUnit tests cover: update allowed when the header matches or is after the server time; 412 when the header is stale after a prior update.

Trac ticket: https://core.trac.wordpress.org/ticket/47676

Use of AI Tools

AI assistance: Yes
Tool(s): Cursor (AI-assisted editing)
Used for: Implementation outline, code changes, and test scaffolding; I reviewed the diff, ran PHPUnit, and adjusted behavior and messages as needed before opening this PR.

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