Fix MCP large result controls

[Widthdom]

Summary

• Add MCP result envelopes with count/truncated/total metadata.
• Add countOnly summary responses for search and graph tools.
• Cap MCP response bytes, stream search snippet compaction, and bound graph aggregate payloads.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedNameMcpServerTests.ToolsCall_Search_IncludesTruncatedAndTotalEnvelope|FullyQualifiedNameMcpServerTests.ToolsCall_References_IncludesTruncatedAndTotalEnvelope|FullyQualifiedNameMcpServerTests.ToolsCall_References_CountOnly_OmitsRowsAndReturnsHistogram|FullyQualifiedNameMcpServerTests.ToolsCall_ImpactAnalysis_CountOnly_OmitsCallerRows|FullyQualifiedNameMcpServerTests.ToolsCall_ResponseOverByteLimit_ReturnsStructuredError|FullyQualifiedNameMcpServerTests.ProcessFrameAsync_BatchResponseOverByteLimit_ReturnsStructuredError|FullyQualifiedName~SearchSnippetFormatterTests.ToCompactResults_IsLazy" -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• codex adversarial review: No blocking/actionable issues found.

Notes

• Added changelog fragments under changelog.d/unreleased/.
• AGENT.md, CLAUDE.md, and AGENT_GUIDE.md were not modified.
• All target issues had no comments indicating someone else had started work.

Fixes #1745
Fixes #1808
Fixes #1732
Fixes #2027
Fixes #2029