test(probes,detectors): add unit tests for dan, promptinject, web_injection

[nuthalapativarun]

What does this PR do

Adds unit test coverage for three modules that currently have no tests:

• detectors.dan.* (DAN, AntiDAN, DANJailbreak, STAN, DUDE, DevMode, MarkdownLink) - verifies
  the DANDetectorMeta-generated StringDetector subclasses are wired with the correct
  substrings, hit on their marker strings, and pass on clean output.
• probes.promptinject.Hijack* / Hijack*Full - verifies prompts are generated via the
  PromptInject framework, primary_detector is wired to promptinject.AttackRogueString,
  capped variants respect soft_probe_prompt_cap, full variants have at least as many prompts
  as capped, and _attempt_prestore_hook sets triggers/settings notes correctly.
• probes.web_injection.* (MarkdownImageExfil, ColabAIDataLeakage, StringAssemblyDataExfil,
  PlaygroundMarkdownExfil, MarkdownURI*ExfilExtended, TaskXSS, MarkdownXSS) - verifies each
  probe loads, generates non-empty prompts, wires the correct primary_detector, and checks
  module-specific behaviour (markdown image markers, prestore hook notes, domain mutation).

This continues the uncovered-module test sweep from #1853.

AI assistance disclosure

Portions of this PR (test scaffolding and initial drafts) were written with AI assistance
(Claude). I reviewed every changed line, ran the new and existing test suites locally, and
confirm the assertions reflect the actual contract/behaviour of the modules under test.

Verification

• Supporting configuration such as generator configuration file - n/a, test-only change
• garak -t <target_type> -n <model_name> - n/a, test-only change
• Run the tests and ensure they pass: python -m pytest tests/probes tests/detectors -k "dan or promptinject or web_injection" - 376 passed
• Verify the thing does what it should - new tests assert prompt counts/caps, substring
  wiring, primary_detector linkage, and detect() scoring for hit/no-hit cases
• Verify the thing does not do what it should not - tests include negative cases (clean
  output should not trigger DAN detectors)
• Document the thing and how it works - test names and assertions describe expected
  behaviour

[nuthalapativarun]

Bumping this — tests are still passing and it's been about 10 days without a look. Let me know if anything needs adjusting.

[nuthalapativarun]

Hi team — following up on PR #1863 (unit tests for dan, promptinject, web_injection). CI green, no reviews yet. Happy to make adjustments if needed. Thanks!