fix(mcp): support Goose discovery handshake (#8037)

## Summary

Goose 1.52 starts MCP connections with `server/discover`. `buzz-dev-mcp`
rejects that request with `ExpectedInitializeRequest`, preventing Goose
from loading its Buzz tools.

Upgrade the tool server to rmcp 3.4.1 and adopt the renamed SDK types.
Subprocess tests exercise discovery and legacy initialization, tool
listing, file reads/edits, invalid arguments, connection survival, and
EOF shutdown. Include the server in both Rust unit-test runners and make
the cancellation regression approve its shell call before testing
cancellation.

The native agent retains its independent rmcp 1.x client. Cargo also
unifies the existing 3.x dependency used by the mesh smoke harness.

## Validation

- Normal `git push` passed all hooks, including Rust tests, Tauri
checks, and repository policy checks.
- `just ci` passed on the final stack: repository checks, Rust and Tauri
tests/lint, desktop tests/build, web build, and mobile checks/tests.
- MCP: 78 existing tests and two subprocess lifecycle tests passed; the
discovery regression fails with the old SDK.
- All 54 native-agent regressions passed on the final stack, including
cancellation of a real MCP shell process.
- Previously exercised unmodified Goose 1.52 and 1.47 with Sandpit and
the real tool server. Allowed workspace operations and permitted network
traffic succeeded; disallowed file and network operations were denied.
- Live staging desktop check with Goose 1.52: the reply appeared in
Buzz, an allowed request returned HTTP 200, and a disallowed request
returned CONNECT 403.

## Stack

1. **#8042 → `main`**: deterministic handoff regression.
2. **This PR → #8042**: Goose MCP discovery compatibility.

Merge the lower PR first, then retarget this PR to `main`. This diff
contains only the compatibility change and its tests; the relay build
repair is already on `main` in #8036.

---------

Signed-off-by: Alex Rosenzweig <arosenzweig@squareup.com>

scripts/run-tests.sh

@@ -110,6 +110,10 @@ run_unit_tests() {
   run_test_step "buzz-acp tests" \
     cargo test -p buzz-acp -- --nocapture
 
+  # Keep MCP lifecycle coverage in step with the nextest path.
+  run_test_step "buzz-dev-mcp tests" \
+    cargo test -p buzz-dev-mcp -- --nocapture
+
 
   # buzz-db migrator/lint unit tests (no infra): guard the embedded-migrator
   # invariant (exactly the consolidated 0001; cutover/backfill stays an operator