Extend RDKit converter with InChI and InChIKey conversions

[Copilot]

Adds offline (compute) conversion routes to the RDKit converter for InChI and InChIKey derivation from SMILES and InChI inputs.

New conversions

• canonical_smiles / isomeric_smiles → inchi — via MolToInchi
• canonical_smiles / isomeric_smiles → inchikey — via MolToInchi + InchiToInchiKey
• inchi → inchikey — via InchiToInchiKey
• smiles → inchi / inchikey — auto-discovered via naming convention

Implementation notes

• smiles_to_inchi and smiles_to_inchikey are registered in the conversions list for canonical_smiles/isomeric_smiles variants; inchi_to_inchikey uses the _to_ naming convention for auto-discovery and is intentionally not registered dynamically (doing so would shadow the class method, causing infinite recursion).
• Both smiles_to_inchikey and inchi_to_inchikey guard against empty/None intermediate values before calling InchiToInchiKey.

rdkit = RDKit()
rdkit.isomeric_smiles_to_inchi("C[C@H](O)c1ccccc1")
# → {"inchi": "InChI=1S/C8H10O/c1-7(9)8-5-3-2-4-6-8/h2-7,9H,1H3/t7-/m1/s1"}

rdkit.inchi_to_inchikey("InChI=1S/C8H10O/...")
# → {"inchikey": "RWMIMWHPCEFZSA-SCSAIBSYSA-N"}

Note: smiles → iupac_name is not implemented here — RDKit has no native IUPAC name generation. The annotation pipeline can already chain smiles → inchi (RDKit) + inchi → iupac_name (IDSM/PubChem) to cover that path.

[hechth]

@copilot please also update the changelog accordingly

[Copilot]

@copilot please also update the changelog accordingly

Done — updated the changelog in commit Update changelog for RDKit InChI/InChIKey conversions to add the new conversions under the [0.5.1] entry.