test: fix vacuous/incorrect indentation tests and remove 3 low-value yaml-only tests in compiler_tests

[github-actions]

Test Suite Reduction: tests/compiler_tests.rs

What was wrong

• test_debug_pipeline_probe_step_indentation_standalone: The for loop made no assertion when the displayName line was absent — the test would silently pass even if the probe step was removed from the output entirely. Additionally, the search string used displayName: "Verify MCP backends" (double-quoted) but the compiled YAML emits the value unquoted (displayName: Verify MCP backends), so the line was never found. The expected indentation constant was also stale: 8 instead of the actual 4.

• test_debug_pipeline_probe_step_indentation_1es: Same two bugs. Expected indent was 18; actual output uses 12.

• test_standalone_complete_compiled_output_is_valid_yaml: Sole body was assert_valid_yaml(compile_fixture("complete-agent.md"), ...) — a single assertion on a fixture that is already compiled and more thoroughly checked by test_standalone_complete_agent_has_setup_and_teardown_jobs.

• test_pr_filter_tier1_compiled_output_is_valid_yaml: Same pattern — only asserted YAML validity; test_pr_filter_tier1_has_evaluator_gate already compiles the same fixture with richer assertions.

• test_pr_filter_tier2_compiled_output_is_valid_yaml: Same pattern.

Changes

Test	Action	Reason
test_debug_pipeline_probe_step_indentation_standalone	Rewritten	Vacuous loop + wrong search string + stale indent constant (8→4)
test_debug_pipeline_probe_step_indentation_1es	Rewritten	Same bugs; stale constant 18→12
test_standalone_complete_compiled_output_is_valid_yaml	Removed	assert_valid_yaml merged into companion test
test_pr_filter_tier1_compiled_output_is_valid_yaml	Removed	assert_valid_yaml merged into companion test
test_pr_filter_tier2_compiled_output_is_valid_yaml	Removed	assert_valid_yaml merged into companion test

Verification

• cargo test: 129 passed, 0 failed ✅
• cargo clippy --all-targets --all-features: no warnings ✅

Warning

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