fix(mcp): restore scalable project discovery

[tmonestudio]

What does this PR do?

Fixes two MCP regressions:

• restores deterministic pagination for list_projects, sorting database names and opening only the requested page before collecting project metadata;
• makes expensive node/edge/Git details opt-in through include_details, while preserving the existing metadata_only=true compatibility path;
• copies normalized check_index_coverage scope strings into the yyjson document so non-root scopes such as src cannot become NUL bytes after the stack buffer is reused.

Reproduce-first evidence

• Production cache (632 databases): current main exceeded 60 seconds for limit=50, include_details=false; this branch returned 50/632 in 3.54 seconds.
• Before the fix, scopes=["src"] serialized scope as three NUL bytes. The regression guard now asserts the exact requested_scope and scope strings.

Checklist

• Every commit is signed off (git commit -s)
• MCP suite passes locally: 166 passed, 6 platform skips
• New behavior is covered by reproduce-first tests
• Promoted-binary smoke passed against the 632-project cache