fix: bound resolver caches, validate MCP input sizes, add integration tests (#213)

Replace the 7 unbounded ReferenceResolver Map caches with a bounded LRU
(env-tunable via CODEGRAPH_RESOLVER_CACHE_SIZE) so memory stays flat on large
codebases, and add length caps on MCP tool string inputs (query/task/symbol +
projectPath/path/pattern) to prevent oversized-payload DoS. Includes LRU,
MCP-input-limit, and full-pipeline integration tests.

Closes #213