Add PYAUTOFIT_TEST_MODE levels 2 and 3 to bypass sampler

[Jammy2211]

Summary

Adds two new test mode levels that bypass the sampler entirely, enabling much faster integration/smoke testing. Level 2 calls the likelihood function once (to verify it works), level 3 skips everything for maximum speed. Both produce the full output folder structure so downstream code works unchanged.

Also centralizes all scattered os.environ.get("PYAUTOFIT_TEST_MODE") checks into a shared test_mode.py module and improves log messages to describe each level's behavior.

Timing on model_fit.py: mode 1 ~71s → mode 2 ~37s → mode 3 ~32s.

Closes #1179

API Changes

Added autofit.non_linear.test_mode module with is_test_mode() and test_mode_level() public helpers. Existing PYAUTOFIT_TEST_MODE=1 behavior is unchanged. New levels 2 and 3 are opt-in via the same environment variable.
See full details below.

Test Plan

• PyAutoFit unit tests: 1198 passed
• PyAutoArray unit tests: 739 passed
• PyAutoGalaxy unit tests: 852 passed
• PyAutoLens unit tests: 286 passed
• Smoke tests with mode 2 and 3: no new failures

Full API Changes (for automation & release notes)

Added

• autofit.non_linear.test_mode.test_mode_level() — returns current test mode level (0-3)
• autofit.non_linear.test_mode.is_test_mode() — returns True if any test mode is active
• AbstractSearch._fit_bypass_test_mode(model, analysis, call_likelihood) — internal method to bypass sampler in modes 2/3
• AbstractSearch._build_fake_samples(model, parameter_vector, log_likelihood) — internal method to build minimal Sample objects for bypass mode
• PYAUTOFIT_TEST_MODE=2 — bypass sampler, call likelihood once
• PYAUTOFIT_TEST_MODE=3 — bypass sampler and likelihood entirely

Changed Behaviour

• All internal os.environ.get("PYAUTOFIT_TEST_MODE") == "1" checks replaced with is_test_mode() which returns True for levels 1, 2, and 3
• Test mode log messages now describe each level's specific behavior

Migration

• No migration needed — existing PYAUTOFIT_TEST_MODE=1 behavior is unchanged
• Downstream repos should import from autofit.non_linear.test_mode import is_test_mode instead of checking the env var directly

🤖 Generated with Claude Code

[Jammy2211]

Workspace PRs:

• Disable LBFGS smoke test for test mode bypass autofit_workspace#20
• Disable ellipse smoke test for test mode bypass autogalaxy_workspace#10
• Disable point_source smoke test for test mode bypass autolens_workspace#33
• Disable visualization smoke tests for test mode bypass autolens_workspace_test#11