BUG: report missing impact roots explicitly

[ting-hong-shieh]

Pull request type

• Code changes (bugfix)
• Code maintenance
• ReadMe, Docs and GitHub updates
• Other

Closes #1147.

Current behavior

Flight.__handle_impact_event() reports multiple valid interpolation roots explicitly, but assumes the list is non-empty before reading its first element.

At merge base cb6106a717207dd8fc2dfe1446d80ff75022f21b, a one-second step whose cubic roots are [-1 + 0j, 2 + 0j] produces:

IndexError: list index out of range

New behavior

The impact handler checks the zero-root case before indexing and reports:

ValueError: No valid roots found when solving for impact time.

This matches the existing rail-exit error style. The single-root path is unchanged, and multiple valid roots retain their existing ValueError.

At head e511a80b2cb0c8140db6818af0dcff2245e217b0, focused tests exercise all three root counts. The single-root case also verifies impact time, velocity, phase updates, time-node updates, and solver termination.

Verification

• .venv/bin/python -m pytest tests/unit/simulation/test_flight.py -k handle_impact_event -q: 3 passed, 60 deselected
• .venv/bin/python -m pytest tests/unit/simulation/test_flight.py -q: 59 passed, 4 skipped
• .venv/bin/ruff check rocketpy/simulation/flight.py tests/unit/simulation/test_flight.py: passed
• .venv/bin/ruff format --check rocketpy/simulation/flight.py tests/unit/simulation/test_flight.py: passed
• git diff --check: passed

The full integration and slow test suites were not run locally.

Environment: RocketPy 1.13.0; Python 3.12.6; NumPy 2.5.2; SciPy 1.18.0; pytest 9.1.1; Ruff 0.16.3; macOS 26.5.2 arm64.

Breaking change

• No

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.29%. Comparing base (e0ff281) to head (e511a80).
⚠️ Report is 53 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1148      +/-   ##
===========================================
+ Coverage    82.18%   83.29%   +1.11%     
===========================================
  Files          122      130       +8     
  Lines        16355    17082     +727     
===========================================
+ Hits         13441    14229     +788     
+ Misses        2914     2853      -61     

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