Improve MCP stdio transport error handling, add unit test, adjust tests and deps

[devopsariful]

Motivation

• Make MCP subprocess startup failures return a handled error instead of panicking and improve diagnostic context for spawn/stdin/stdout failures.
• Ensure unit tests cover the failure mode for MCP stdio transport and make integration tests more deterministic about expected results.
• Normalize a few doc/test spellings and align dependency versions for compatibility.

Description

• Change StdioTransport::new to return Result<Self> and use anyhow::Context for richer error messages when spawning the subprocess or capturing stdin/stdout instead of unwrapping or panicking.
• Update MCPClient::register_server to propagate the Result from StdioTransport::new (use ?) so callers handle transport construction failures.
• Add a unit test stdio_transport_spawn_failure_returns_error to verify that attempting to spawn a non-existent MCP command returns an error.
• Adjust integration tests (test_vector_store and test_config) to use US English spellings and make assertions more precise by comparing PathBuf values directly; update README.md test docs accordingly.
• Bump/adjust package dependency versions in package.json (notably @milkdown/plugin-math, @milkdown/plugin-table, and @tauri-apps/api) and include a small Cargo.lock change for thiserror.
• Minor comment wording change in voice.rs (British → American spelling) and small tidy-ups to error handling code paths.

Testing

• Ran cargo test which includes unit tests and integration tests, and the test suite completed successfully including the new stdio_transport_spawn_failure_returns_error unit test.
• Verified test_vector_store and test_config integration tests ran as part of cargo test and passed after the test assertion adjustments.
• No runtime/manual UI tests are included in this change; CI should run the full test matrix to confirm cross-platform behaviour.

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Codex Task