Replace panic with Secp256k1 variant in HpkeError

[DanGould]

Summary

• Added Secp256k1(secp256k1::Error) variant to HpkeError to replace the panic! catch-all in From<secp256k1::Error>
• Updated Display and Error trait implementations for the new variant
• Added test verifying that non-InvalidPublicKey variants convert without panicking

Closes payjoin#1274

Test plan

• nix fmt -- --ci passes
• cargo clippy --all-targets --keep-going --all-features -- -D warnings clean
• codespell clean
• CI passes on fork

Disclosure: co-authored by Claude

[coveralls]

Pull Request Test Coverage Report for Build 22572334991

Details

• 9 of 11 (81.82%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 82.49%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
payjoin/src/core/hpke.rs	9	11	81.82%

Totals
Change from base Build 22572195695:	0.006%
Covered Lines:	10633
Relevant Lines:	12890

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💛 - Coveralls

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