Skip reports coverage when no PHP tests are available

[coisa]

Problem

dev-tools reports always launches dev-tools tests as a mandatory sub-command. In repositories that do not have PHPUnit tests and may only contain docs/assets, reports execution can fail the CI path even when no test surface exists (observed in agents repo PR2).

Proposed Behavior

• reports must not fail when test execution is not applicable.
• For repositories without detectable tests, it should skip coverage report generation and continue generating the remaining report surfaces (docs/metrics).
• The command must preserve a successful outcome when docs and metrics are produced and tests are skipped.
• If tests exist but fail, reports should continue to fail as today.

Acceptance Criteria

• reports behavior:
  • when no tests/ directory and no testable PHP source is detected, coverage/report test generation is skipped.
  • logs/structured output should clearly indicate that tests were skipped because no runnable test surface was detected.
• dev-tools reports in a repository with no PHP tests should exit 0.
• Existing behavior remains unchanged when tests are available.
• Add/adjust unit tests for this branch:
  • skip tests queueing in ReportsCommand when no runnable tests are available.
  • keep queueing tests when tests are available.
• Update changelog (Unreleased, Fixed) to describe the behavior.

Non-goals

• Detecting partially misconfigured PHPUnit setups beyond existing test-runner behavior.
• Changing tests command internals unless required by docs/report orchestration needs.