feat: add History tab to resume prior agent sessions from disk @W-22434464@

[marcelinollano]

What does this PR do?

Adds a new History tab to the Agentforce DX webview that lists prior cached agent sessions and lets you pick one back up where you left off.

• Reads sessions from .sfdx/agents/<storageKey>/sessions/, the same on-disk format used by the sf CLI plugin, so the list works regardless of which tool wrote the session.
• Each row shows a sessionType badge (Simulation / Live Test), the first user message, and a compressed timestamp; click resumes via SDK agentBase.resumeSession(sessionId) and switches to Preview.
• If the clicked session is already active for the same agent, no restart fires.
• Empty/loading states match the Tracer placeholder layout, including the Start Simulation / Live Test / Send a message button.

What issues does this PR fix or reference?

@W-22434464@ — [AFDX] [UX] Navigate history

Functionality Before

The webview only ever surfaced the most recent session. Switching agents or restarting wiped chat/tracer state with no way to revisit prior runs that were already on disk.

Functionality After

• New History tab next to Preview / Tracer with a clock icon.
• Resume reuses the existing agent instance, restores transcript + traces, and skips the canned welcome message.
• Optimistic UI: clicking a row immediately shows a "Resuming session..." spinner and disables input — no flicker through the "no session" toolbar state.

Testing Setup Notes

1. Open a project with an authoring bundle.
2. Start a simulation, send a few messages, end the session.
3. Open the new History tab — the just-finished session appears, newest first, with badge + first-message + timestamp.
4. Click another row → it resumes that session in place; click the same row again → no-op.
5. Repeat with a Live Test and a Published agent — badges reflect the session type.
6. Delete or rename a script bundle then attempt to resume one of its sessions — a friendly error appears.

Tests

• New sessionHistoryService.test.ts (5 cases) covers the disk-read fallback chain.
• Extended sessionManager.test.ts with resumeSession happy path, end-previous, error, and no-webview cases.
• Extended webviewMessageHandlers.test.ts with listSessions (success/empty/throw) and resumeSession (forward/short-circuit/missing sessionId).
• New SessionHistory.test.tsx covers placeholder, list rendering, empty state with start button, and click-to-resume.

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