[M2] Implement textDocument/hover

[zacky1972]

Summary

Expose a minimal hover query API for Lean source positions.

Tasks

• Build textDocument/hover requests.
• Convert line/character positions to LSP format.
• Parse hover responses.
• Add an integration test against the fixture project.

Acceptance criteria

• LeanLsp.hover(session, path, line, character) returns structured hover information.
• Hover queries work through the Docker-backed runtime.