Add RDKit formula conversions for canonical/isomeric SMILES

[Copilot]

This PR adds the missing RDKit compute conversion paths from SMILES variants to molecular formula. The issue requested support for smiles, canonical_smiles, and isomeric_smiles → formula in the compute converter.

• RDKit conversion routing

  • Extended RDKit.__init__ conversion map to register:
    • canonical_smiles_to_formula
    • isomeric_smiles_to_formula
  • Both routes reuse existing smiles_to_formula logic (no duplicate formula computation path).

• Coverage for new conversion entry points

  • Updated tests/test_rdkit.py with focused cases validating:
    • canonical_smiles_to_formula(...) -> {"formula": ...}
    • isomeric_smiles_to_formula(...) -> {"formula": ...}

conversions = [
    ("smiles", "mw", "from_smiles"),
    ("canonical_smiles", "mw", "from_smiles"),
    ("isomeric_smiles", "mw", "from_smiles"),
    ("canonical_smiles", "formula", "smiles_to_formula"),
    ("isomeric_smiles", "formula", "smiles_to_formula"),
]

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