feat(test): add --changed and --related flags to deno test

[lunadogbot]

Closes #28182

Adds Vitest-style dependency-aware test selection to deno test as a one-shot
(non-watch) run. The issue asked for a way to run only the tests affected by a
change once — watch mode already does dependency-aware reruns, but continuously,
which isn't the workflow the reporter wanted.

Flags

• --changed[=<ref>] — run only test modules affected by files changed in
  git. With no value it uses the working tree (staged, unstaged and untracked);
  with a ref (--changed=main, --changed=HEAD~1) it additionally includes
  committed changes since the merge-base (<ref>...HEAD three-dot form),
  matching Vitest and Jest.
• --related=<files> — run only test modules that depend on the given
  source files, without consulting git (Vitest's --related).

$ deno test --changed                  # tests affected by uncommitted changes
$ deno test --changed=origin/main      # tests affected since branching off main
$ deno test --related=src/util.ts      # tests that import src/util.ts

How it works

Both flags resolve a set of changed/related file paths and then filter the
collected test specifiers, reusing the module graph and the existing
has_graph_root_local_dependent_changed walk that watch mode already uses to
find dependents. Because Deno builds a single module graph spanning the whole
workspace, selection works across workspace members — a test in one member
that imports another member's changed source is selected automatically.

Design details, mostly following prior art (Vitest/Jest/Nx/Turborepo):

• A module that is itself a changed/related file is always kept.
• A changed .env file disables filtering (any test may read it), mirroring the
  existing watch-mode escape hatch.
• Deleted files can't be resolved in the graph and are dropped (their former
  dependents fail typecheck anyway).
• Both flags conflict with --watch (watch already does continuous
  dependency-aware reruns).
• When nothing is affected the run exits 0 with a message instead of erroring,
  matching Vitest's behavior with --changed.

Changes

• cli/args/flags.rs — flag definitions, parsing, TestFlags::{changed,related}
• cli/tools/test/mod.rs — git change detection (changed_files_from_git),
  collect_changed_paths, filter_specifiers_by_changed, wired into run_tests
• tests/specs/test/{related,changed} — spec tests (hermetic --related;
  git-based --changed)

Test plan

• cargo build / cargo clippy (clean under -D clippy::all) / rustfmt
• Unit parse tests for --changed, --related, and the --watch conflict
• Manual end-to-end in real git repos: uncommitted --changed,
  --changed=<ref>, --related, clean-tree (no-affected message),
  cross-workspace (editing b/mod.ts ran a/a_test.ts + b/b_test.ts),
  and self-changed test file
• --related spec output validated byte-for-byte against the built binary
• CI green

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