PublicKey.used_explicit_encoding() in python wrapper

[reneme]

This API is meaningful only for ECC keys and returns true if the loaded public key contained an explicitly defined ECC group.

[Copilot]

Pull request overview

This PR extends the Python Botan 3 wrapper with an API to detect whether an ECC public key uses an explicitly encoded group and adds tests around that behavior.

Changes:

• Adds a new FFI binding for botan_pubkey_ecc_key_used_explicit_encoding in botan3.py.
• Exposes this via PublicKey.used_explicit_encoding() in the Python wrapper, raising a BotanException for non‑ECC keys.
• Extends test_rsa_load_store and test_ecdh to verify the behavior for RSA keys, ECC keys with explicit groups, and standard named ECC groups.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/python/botan3.py	Binds the new FFI function and introduces PublicKey.used_explicit_encoding() with appropriate error handling for non‑ECC keys.
src/scripts/test_python.py	Adds unit tests covering used_explicit_encoding() for RSA keys (exception), an explicitly-parameterized ECC public key, and normal named ECDH groups (returns False).

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[coveralls]

coverage: 90.076% (+0.005%) from 90.071%
when pulling cc86ce8 on Rohde-Schwarz:feature/pubkey_expl_enc
into d951175 on randombit:master.