[codex] Harden MCP audit log bounds and redaction

[Widthdom]

Summary

• Reject excessive --audit-log-max-bytes values above the documented 1 GiB cap.
• Redact secret-like MCP audit arg_values keys and token-shaped values when include-values is enabled.
• Budget include-values payloads and top-level property names before writing audit records.
• Cap audit event key/request-id surfaces and fall back when records exceed the event byte budget.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build --filter "FullyQualifiedName~AuditLogSinkTests|FullyQualifiedName~McpAuditLogTests|FullyQualifiedName~McpServerTests.ProcessLineAsync_CapsTelemetryArgumentKeyCount|FullyQualifiedName~ProgramRunnerTests.TryConsumeAuditLogFlags_MaxBytes"
• Codex adversarial review also ran dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-restore --filter "AuditLogSinkTests|McpAuditLogTests|McpServerTests|ProgramRunnerTests" successfully.
• dotnet run --no-restore --project tools/CodeIndex.Changelog -- check
• git diff --check origin/main..HEAD
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --rebuild --yes --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: final pass found no new blocking/actionable correctness issues.

Fixes #3067
Fixes #3106
Fixes #3237
Fixes #3180