Allow ACP sessions with host-provided MCP servers

[1jehuang]

Summary

• accept and ignore valid ACP mcpServers arrays during session creation and loading
• continue rejecting malformed non-array values
• add regression coverage for non-empty host-provided server arrays

Verification

• cargo test -p jcode --lib cli::acp::tests (16 passed)
• built the PR binary and successfully ran ACP initialize plus session/new with a non-empty mcpServers array over stdio; session creation returned a real sessionId
• GitHub CI: macOS, Windows, Windows cross-target, PowerShell, TypeScript SDK, setup, release, linked-issue, and Greptile checks pass
• Linux is blocked by the pre-existing pinned_todos_payload_stays_empty_when_config_off failure tracked in CI failure: pinned_todos_payload_stays_empty_when_config_off (stale config cache leak from sibling test) #877; Format/Quality are blocked by pre-existing desktop formatting on master

Fixes #887

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— Jcode agent (automated triage), on behalf of @1jehuang

[greptile-apps]

Greptile Summary

This change lets ACP hosts send mcpServers arrays when creating, loading, or resuming a session, while continuing to reject malformed non-array values. The compiled ACP CLI was exercised through its stdin/stdout JSON-RPC interface for all three session routes: non-empty arrays completed normally, and scalar values consistently returned invalid-parameters responses. No defects were found.

Confidence Score: 5/5

The change is safe to merge based on end-to-end ACP request validation across session creation, loading, and resumption.

The actual compiled CLI accepted non-empty arrays on every affected route, rejected invalid scalar values without crashing, and the focused regression test passed.

Files Needing Attention: No files need further attention; src/cli/acp.rs received focused runtime and unit-test coverage.

T-Rex Logs

What T-Rex did

• Ran the ACP JSON-RPC validation script with scalar mcpServers across session/new, session/load, and session/resume, and observed -32602 errors without crashes.
• Tested non-empty mcpServers arrays across the same routes; session/new returned a session result and session/load/resume produced normal results with command-update notifications.
• Executed the focused ACP mcpServers unit test; it passed.
• Validated cross-route behavior for invalid scalar and non-empty arrays with added log evidence; all three routes returned -32602 for invalid scalar, and non-empty arrays were accepted and treated as ignored MCP commands.

_{Ran code and verified through T-Rex}

_{Reviews (1): Last reviewed commit: "fix: tolerate ACP mcpServers during sess..." | Re-trigger Greptile}