test: reduce and improve tests in src/validate.rs

[github-actions]

Test Suite Reduction: src/validate.rs

What was wrong

• test_reject_pipeline_injection: Two consecutive assertions both tested the contains_template_marker branch — \{\{ agent_content }} and \{\{ copilot_params }} trigger exactly the same code path. The second assertion was a duplicate; meanwhile the $[...] ADO runtime expression form was never exercised through reject_pipeline_injection.
• test_validate_container_image: Both invalid-input branches were checked only with .is_empty(). The warning message itself — which contains the specific reason (empty name / unexpected characters) — was never asserted. A regression that changed the message to a different string would go undetected.
• test_validate_docker_args_volume_flag_calls_mount_validation: The test asserted warnings.len() >= 2 and checked warnings[0] but never verified warnings[1], so the sensitive-path sub-warning from validate_mount_source was effectively untested.
• test_validate_mcp_url: The invalid-URL case was checked only with .is_not_empty(). The warning message — which explains which schemes are acceptable — was never asserted.
• test_validate_feed_url_rejects_double_quote + test_validate_feed_url_rejects_single_quote: Two separate test functions each containing a single assertion for the same feature (quote rejection in feed URLs). Unnecessarily split.

Changes

Test	Action	Reason
test_reject_pipeline_injection	Rewritten (one assertion replaced)	Duplicate template-marker assertion → add untested $[variables.x] runtime expression form
test_validate_container_image	Rewritten	.is_empty() only → verify warning message content for both invalid cases
test_validate_docker_args_volume_flag_calls_mount_validation	Rewritten	Missing assertion on warnings[1] sensitive-path content
test_validate_mcp_url	Rewritten	.is_empty() only → verify warning message mentions expected schemes
test_validate_feed_url_rejects_double_quote + test_validate_feed_url_rejects_single_quote	Merged → test_validate_feed_url_rejects_quotes	Two single-assertion tests for the same feature

Verification

• cargo test: 1602 tests pass, 0 failed ✅
• cargo clippy --all-targets --all-features: no new warnings ✅

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