Guard code extraction sessions and reset UI before extraction

[rawritude]

Motivation

• Prevent concurrent extractCode() calls from colliding and producing stale UI updates.
• Ensure the UI shows a clear "Loading..." state and cleared code before starting extraction.
• Queue follow-up extractions triggered while one is in-flight to avoid lost updates.
• Only apply extraction results for the most recent session.

Description

• Added session tracking and an in-flight guard using isExtracting, extractSessionId, latestExtractSessionId, and pendingExtractSessionId to serialize extractions.
• Split extraction into extractCode() and runExtractCode(sessionId) and ignore responses where sessionId !== latestExtractSessionId to prevent stale updates.
• Added prepareCodeExtractionUI() to reset projectCode, codeDisplay, and set codeStatus to Loading... before kicking off extraction.
• Wire workspaceSessionChanged messages to trigger extractCode() so the panel refreshes when the workspace changes.

Testing

• No automated tests were run for this change.
• Manual runtime checks were used during development (no test failures reported).
• The repository's existing test suite was not executed as part of this change.
• Changes were committed and verified to apply cleanly to sidepanel/sidepanel.js.

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Codex Task