fix: respect gitignore in marker walk, classify MCP errors by fault

[bug-ops]

Summary

• standard_filters(false) negates the preceding git_ignore(true) in the project-marker walk #476 — LspServerConfig::find_any_marker_recursive called WalkBuilder::standard_filters(false) after git_ignore(true), silently negating it (last write wins) and letting the project-marker search descend into gitignored directories. Reordered so standard_filters(false) runs first.
• map_bridge_error reports every caller-fault error as -32603 internal_error #479 — map_bridge_error routed nearly every crate::error::Error variant through the generic -32603 INTERNAL_ERROR, including clear caller-fault variants, while the resource handlers independently reimplemented correct -32602 INVALID_PARAMS classification for a subset of cases. Added an exhaustive Error::mcp_error_kind() classification (no wildcard arm) and routed both map_bridge_error and the resource handlers through it, removing the duplicated .map_err(...) chains.

Classification notes (from review)

• Caller-fault (invalid_params, -32602): InvalidToolParams, PathOutsideWorkspace, NotARegularFile, InvalidUri, DocumentNotFound, FileSizeLimitExceeded, and FileIo when caused by a nonexistent path (ErrorKind::NotFound).
• Retryable (bespoke code + structured data): WorkspaceIndexing (existing, -32050), ServerInitializing and WorkspaceServersInitializing (new, -32051) — both represent "expected LSP server hasn't registered yet."
• Internal (-32603): NoWorkspaceRoots (workspace roots are a startup-fixed snapshot — an empty set never fills, no retry helps) and DocumentLimitExceeded (aggregate tracker state, not a bad property of the specific request — the identical request can succeed later once other documents close).

Test plan

• cargo +nightly fmt --all -- --check
• cargo clippy --all-targets --all-features --workspace -- -D warnings
• cargo nextest run --workspace --all-features --lib --bins (973 passed)
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --all-features
• cargo test --doc (28 passed)
• New regression test for standard_filters(false) negates the preceding git_ignore(true) in the project-marker walk #476 verified to fail under the pre-fix WalkBuilder ordering
• New tests pin the JSON-RPC error code for each reclassified Error variant, including through the read_resource/subscribe handler chain

Closes #476
Closes #479