Add sandbox TTL edge-case tests

[lpc0387]

5. Add sandbox TTL edge-case tests

Labels: good first issue, tests, sandbox

Body:

The sandbox launcher supports custom TTL values such as 0.5 hours and defaults to an auto-stop timeout. Add targeted tests or script checks for TTL parsing edge cases that are easy to reason about and do not start long-lived processes.

Suggested scope:

• Inspect existing sandbox tests before choosing the exact test location.
• Cover one or two edge cases such as fractional hours and invalid values.
• Prefer testing parsing/helper behavior over launching a real server.
• Keep the test fast and deterministic.

Acceptance criteria:

• The added coverage proves a valid fractional TTL is accepted or represented correctly.
• The added coverage proves an invalid TTL is rejected or handled with the documented error path.
• The verification command is narrow, such as PYTHONPATH=src python3 -m unittest tests.test_sandbox_ttl -v.
• The test does not leave behind sandbox processes, databases, logs, or workspace files.

Why it is newcomer-friendly:

The behavior is concrete and already documented. The contributor can focus on one existing test area instead of learning the full engine.

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This issue was created from the v5.0 launch backlog in docs/launch/good-first-issues.md.