What does this PR do?

Optimizes CI Visibility impacted-test analysis by caching IsImpacted decisions per analyzer and replacing per-call temporary test bitmap allocation with a direct bitmap range intersection check.

The implementation preserves existing behavior:

• suffix-based modified-file matching
• first matching modified file wins
• missing line information marks matching files as impacted
• existing debug log fragments are still emitted, including on cache hits
• analyzer state is safe for concurrent IsImpacted calls

It also adds benchmarks for repeated cached calls, cold distinct ranges, misses, missing-line-info decisions, and the direct bitmap range check.

Motivation

Profiling after the previous CI Visibility optimizations showed IsImpacted still doing repeated work on retries/subtests. The previous flow rebuilt a temporary bitmap for each test range and scanned modified files on every call. This change keeps the same decisions while avoiding that repeated allocation and memoizing repeated source range lookups.

Latest local benchmark sample:

BenchmarkIsImpactedRepeatedSameRange-12          36.02 ns/op    0 B/op    0 allocs/op
BenchmarkIsImpactedRepeatedDistinctRanges-12     41.69 ns/op    0 B/op    0 allocs/op
BenchmarkIsImpactedColdDistinctRanges-12        533.9 ns/op   173 B/op    3 allocs/op
BenchmarkIsImpactedRepeatedMiss-12               27.41 ns/op    0 B/op    0 allocs/op
BenchmarkIsImpactedNoLineInfo-12                 35.36 ns/op    0 B/op    0 allocs/op
BenchmarkBitmapIntersectsLineRange-12            2.953 ns/op    0 B/op    0 allocs/op

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running make lint locally.
• New code doesn't break existing tests. You can check this by running make test locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• All generated files are up to date. You can check this by running make generate locally.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild. Make sure all nested modules are up to date by running make fix-modules locally.

Local verification:

git diff --check
go test ./internal/civisibility/utils/impactedtests -count=1
go test -race ./internal/civisibility/utils/impactedtests -count=1
go test ./internal/civisibility/utils/... -count=1
go test ./internal/civisibility/integrations/... -count=1
go test ./internal/civisibility/utils/impactedtests -bench='BenchmarkIsImpacted|BenchmarkBitmapIntersects' -benchmem -run '^$'

Notes:

• System tests are not added because this is an internal performance refactor with no agent protocol or user-facing behavior change.
• make lint, make test, make generate, and make fix-modules were not run directly; the narrower local commands above were run against the affected CI Visibility packages, and no generated files or module files changed.