fix: recover from session start failures in the AFDX agent preview @W-22434854@

[marcelinollano]

What does this PR do?

Fixes recovery from session start failures in the AFDX agent preview.

• Title-bar Refresh and Reset (clear-all) icons no longer get stuck after a session start failure. The webview's isSessionStarting and the agentforceDX:sessionStarting context flag are cleared on error, compilationError, clearMessages, and refreshAgents.
• Compilation errors now render the same error screen as other failures (icon + bold title + muted subtitle + Go Back button) instead of plain text inside the chat transcript.
• Error titles are short and distinct: "Agent deactivated", "Agent not found", "Permission denied", "Compilation failed", "Something went wrong". The subtitle holds the human-readable explanation for known cases or the raw server error for the generic case.
• The agent selector stays mounted while the error screen is showing, so clicking Go Back returns to a populated dropdown with controls enabled.
• Multi-line compiler output is preserved in the subtitle (white-space: pre-wrap).

What issues does this PR fix or reference?

@W-22434854@

Functionality Before

• Compilation errors showed as a centered system message with no recovery affordance.
• Other failure types used full-paragraph bold titles, with the raw server error string shown as the subtitle.
• After clicking Go Back from the error screen, the dropdown briefly re-loaded and the top start button stayed disabled because the selector remounted from scratch.
• The title-bar Refresh and Reset icons stayed hidden after early session-start failures.

Functionality After

• All failure types render the same error screen with short titles and explanatory subtitles.
• Refresh and Reset icons are reachable from the error screen and recover the view to a usable state.
• Clicking Go Back keeps the agent selector populated and its controls enabled.
• Compilation error subtitles render multi-line compiler output correctly.

Testing Setup Notes

Automated

• npm run test:backend (381 tests) and npm run test:frontend (387 tests) pass. Updated assertions in test/views/agentCombined/handlers/webviewMessageHandlers.test.ts and test/webview/AgentPreview.messages.test.tsx.

Manual verification

1. Open an agent script project in the Extension Development Host. Select an agent that has a syntax error and click Start Simulation. Expected: error screen with icon, "Compilation failed" title, multi-line compiler output as subtitle, Go Back button. Title-bar Refresh and Reset icons are visible.
2. From step 1, click Go Back. Expected: returns to the agent selector with the same agent selected, dropdown not loading, top start button enabled.
3. From step 1, click the title-bar Refresh icon instead. Expected: agents list reloads and the view returns to the placeholder.
4. From step 1, click the title-bar Reset icon. Expected: clears the error and returns to the agent placeholder for the same agent.
5. Authenticate to an org and select a deactivated agent, then Start Live Test. Expected: title "Agent deactivated" with the activation guidance as the subtitle.
6. Repeat with a 403/permission scenario and a 404/deleted agent. Expected: titles "Permission denied" and "Agent not found" with the matching explanation.
7. Successful session start regression: select a healthy agent and Start Simulation. Expected: no error screen flashes; chat loads; stop and restart work.

[npiccolo]

Manual testing ✅

Setup: Extension Development Host launched from ml/W-22434854-buttons-stuck-on-session-failure, agent script project afdx-pro-code-testdrive opened in the test window.

Compilation error recovery
Selected TestVerboseAgent, introduced a syntax error, clicked Start Simulation. Error screen appeared with "Compilation failed" as the bold title and the compiler output as the subtitle. Title-bar Refresh and Reset icons were visible and responsive. Clicking Go Back returned to the agent selector with the agent still selected, dropdown not loading, Start button enabled and ready.

Happy path regression
Restored the agent file, selected TestVerboseAgent, clicked Start Simulation. Chat loaded cleanly with no error flash. Sent a message and received a correct response. Clicked Stop then Start Simulation again — restarted cleanly with no stuck state.

Automated tests: npm run test:backend (381/381) and npm run test:frontend (387/387) all pass.

LGTM ✅