Add generate_thermo_ensemble: charge-state-aware conformers with auto-retry

[galjos]

Building a conformer ensemble for a charged species (radical anion, dianion -- the exact pattern `pKa`/`reduction_potential`/`EnsembleThermo` exist for) currently means hand-rolling a loop around `conformers.generate()` plus a `Thermo` engine, manually reseeding whenever a conformer optimizes onto a saddle point instead of a true minimum (`TSValueError`) or the engine's CLI process fails outright (`RuntimeError`).

The fix

`generate_thermo_ensemble(smiles, thermo_fn, charge=0.0, max_conformers=10, max_attempts=5, ...)` does that loop once: generate conformers, compute `Thermo` for each via a caller-supplied engine callable (e.g. `xtb_cli_thermo`), skip conformers that raise `TSValueError`/`RuntimeError`, and reseed+retry (up to `max_attempts` distinct random seeds) until enough succeed. Returns a plain list of `Thermo`, ready for `EnsembleThermo`.

Engine-agnostic by design (like the rest of the pKa/redox/ensemble helpers) -- `thermo_fn` is just called as `thermo_fn(atoms, charge=charge, **thermo_kwargs)`, so it works with `xtb_cli_thermo`, `dftbplus_thermo`, or any custom callable with that signature.

Verification

Unit tests cover: respecting `max_conformers`, forwarding `charge`/kwargs, skipping+retrying past both failure types, stopping mid-attempt once the target is reached (not over-computing), raising a clear `ValueError` when nothing succeeds within `max_attempts`, and NOT swallowing unrelated exceptions. A real end-to-end test (skippable, gated on the native `xtb` binary, matching the existing pattern) builds a real ensemble for 4-hydroxybutanoic acid's anion and feeds it into `EnsembleThermo`.

Full suite: 410 passed, 10 skipped with a real `xtb` binary. 98% coverage on `conformers.py` (the one uncovered line pre-dates this change). Docs build clean.

Closes #120

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.34%. Comparing base (0636de4) to head (caf56e3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #121      +/-   ##
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+ Coverage   97.31%   97.34%   +0.02%     
==========================================
  Files          32       32              
  Lines        1940     1957      +17     
==========================================
+ Hits         1888     1905      +17     
  Misses         52       52              

Flag	Coverage Δ
unittests	97.34% <100.00%> (+0.02%)	⬆️

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