Bound MCP environment override limits - #2950

Summary

• Clamp MCP response and batch response byte-limit environment overrides, with effective values exposed in MCP status.
• Clamp MCP graph pagination offset before SQL queries and advertise the cap in tools/list / status.
• Reject non-finite and out-of-range MCP HTTP keep-alive intervals.
• Clamp MCP rate limiter RPS/burst overrides and expose effective limiter settings in status.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-restore --filter "FullyQualifiedName~McpServerTests|FullyQualifiedName~RateLimiterTests|FullyQualifiedName~HttpMcpTransportTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• git diff --check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --files ... --json

Adversarial review covered oversized env values, non-finite keep-alive/rate inputs, SQL offset clamping before query execution, and status visibility.

A full dotnet test CodeIndex.sln --no-restore run surfaced intermittent existing SQL reference test failures that passed when rerun individually; filed #2949.

Fixes #2861
Fixes #2895
Fixes #2899
Fixes #2900

Related: #2949