fix(slack): avoid per-channel metadata lookups

[akshaan]

Summary

• replace sequential conversations.info calls with a cached, paginated conversations.list catalog for public-channel metadata
• intersect the catalog with the exact channel IDs authorized by the caller JWT
• preserve the existing per-channel lookup for private, DM, or otherwise missing channels
• add focused pagination and authorization regression tests

Root cause

The Slack channels endpoint fetched metadata sequentially for every permitted channel. A User MCP role with 909 public channels could not finish within the Slack client timeout, so fallback search timed out before it could scan channel history.

Permission behavior

This does not broaden channel access. The endpoint still returns only channel IDs present in the caller JWT claims; the workspace-wide public catalog is used only to enrich those already-authorized IDs with metadata. Missing IDs retain the existing conversations.info path.

Validation

• cargo fmt --all --check
• cargo clippy -p centaur-api-server --all-targets -- -D warnings
• cargo test -p centaur-api-server slack_proxy::tests
• cargo test -p centaur-api-server