Fix unused analysis for partial-class handlers and field-like symbols

[Widthdom]

Found while dogfooding the locally built cdidx on branch codex/search-audit-issues-20260619.

Duplicate check: no open issue found for unused partial class false positives MCP handlers constants fields kind function.

Evidence:

• cdidx unused --json --exclude-tests --visibility private flags src/CodeIndex/Mcp/McpToolDefinitions.cs:17 HandleToolsList and large MCP handlers such as src/CodeIndex/Mcp/McpToolHandlers.cs:3814 ExecuteBatchQuery.
• It also reports field-like symbols such as private constants and readonly fields with kind: "function", for example src/CodeIndex/Mcp/McpServer.cs:160 MaxLimit.

Problem:
These look like likely false positives or metadata-shape issues in unused, especially around partial classes and C# field/constant classification.

Acceptance criteria:

• Teach unused analysis to resolve references across partial class files reliably.
• Emit accurate symbol kinds for C# constants/fields/properties in unused output.
• Add regression tests that keep active MCP handlers from being reported as unused.

[Widthdom]

Work started on branch fix-issue3673-3687-3692-3697-3779.