feat(rate-limiter): add per-server user limits

[vishu-bh]

Summary

• add optional by_user_per_server rate limits keyed by user and MCP server
• keep by_user as the global per-user ceiling across servers
• keep by_tenant as an optional shared infrastructure ceiling
• validate server identifiers and fail closed when the configured dimension is unavailable
• support memory and Redis backends across fixed-window, sliding-window, and token-bucket algorithms
• update manifest, Python API/stubs, documentation, tests, and package version to 0.1.11

Why

A shared tenant bucket does not provide fair allocation between users. One user can consume nearly all shared capacity. The new dimension gives every user an independent allowance for each server while preserving the existing global user limit.

Testing

• cargo test -p rate_limiter — 89 passed
• cargo clippy -p rate_limiter --all-targets -- -D warnings
• focused Python integration suite — 61 passed
• real Redis replica/isolation test — passed
• cargo fmt --all -- --check
• git diff --check

The umbrella integration run reached the existing Redis TLS fixture but could not complete locally because its generated certificate was rejected for a missing Authority Key Identifier. The focused feature suites and real Redis isolation test pass.

Closes #172.

[gandhipratik203]

LGTM!

[lucarlig]

LGTM