Fix find context controls and regex mode

[Widthdom]

Summary

• Clamp MCP excerpt/find context values instead of rejecting large before/after values.
• Add find snippet, focus-line/focus-column, and regex controls for CLI and MCP.
• Keep find count mode aligned with regex/focus semantics and return user-facing regex errors for invalid or timed-out regexes.

Issues

Fixes #1413
Fixes #1414
Fixes #1593
Fixes #1597
Fixes #1930

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedNameQueryCommandRunnerTests.RunFind_SnippetLinesControlsMatchContext|FullyQualifiedNameMcpServerTests.ToolsCall_FindInFile_SnippetLinesControlsMatchContext|FullyQualifiedNameQueryCommandRunnerTests.RunFind_FocusLineAndColumnRestrictMatch|FullyQualifiedNameMcpServerTests.ToolsCall_FindInFile_FocusLineAndColumnRestrictMatch|FullyQualifiedNameQueryCommandRunnerTests.RunFind_RegexMatchesAnchors|FullyQualifiedNameQueryCommandRunnerTests.RunFind_CountOnlyRegexAndFocusUseSameMatchingSemantics|FullyQualifiedNameMcpServerTests.ToolsCall_FindInFile_RegexMatchesAnchors|FullyQualifiedNameMcpServerTests.ToolsCall_Excerpt_BeforeAboveCapClampsContext|FullyQualifiedNameMcpServerTests.ToolsCall_Excerpt_AfterAboveCapClampsContext|FullyQualifiedNameMcpServerTests.ToolsCall_FindInFile_BeforeAfterAboveCapClampContext"
• dotnet run --project tools/CodeIndex.Changelog -- check

Review

• adversarial review found count-mode regex/focus drift and regex timeout handling; both were fixed in follow-up commit 8b58235.
• A second adversarial-review run was attempted after the fix, but Codex returned a usage-limit error before reviewing.