feat(internal/civisibility): subtest-level test management & flaky retry support.

[tonyredondo]

What does this PR do?

• Introduces subtest-level test-management and attempt-to-fix orchestration for the Go testing integration, plumbing hierarchical identities, ownership rules, and metadata propagation as described in docs/SUBTEST_FEATURE_IMPLEMENTATION.md.
• Adds a dedicated subtest scenario harness under integrations/gotesting/subtests that spins up a mock backend, drives parent/subtest directive matrices, and validates span/tag output (including parallel and custom-retry cases).
• Documents the new architecture in CIVISIBILITY_OVERVIEW.md, giving contributors a map of the identity plumbing, feature gating, and harness layout.
• Registers DD_CIVISIBILITY_SUBTEST_FEATURES_ENABLED with the stable configuration allowlist so the new feature flag is recognized by the tracer.

Motivation

We needed first-class support for Datadog Test Management directives at the subtest level (t.Run), with predictable retry ownership and tagging that mirrors parent behaviour.

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 ./scripts/lint.sh locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-10-24 08:05:37

Comparing candidate commit 36770a1 in PR branch tony/subtest-ancilliary-features-v1 with baseline commit 789b238 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 24 metrics, 0 unstable metrics.

[datadog-official]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

_{This comment will be updated automatically if new data arrives.
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