Add safe catch-all for secp256k1::Error conversion in HpkeError

[Mshehu5]

Closes: #1274

Replace panic with a match to Other(String) to handle unknown secp256k1 error variants. This prevents crashes for unknown varients and if new variants are added upstream.

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[coveralls]

Pull Request Test Coverage Report for Build 21340164710

Details

• 17 of 20 (85.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 83.213%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
payjoin/src/core/hpke.rs	17	20	85.0%

Totals
Change from base Build 21287723795:	0.01%
Covered Lines:	10157
Relevant Lines:	12206

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💛 - Coveralls

[benalleng]

cACK seems simple enough, not sure if there was another approach in mind but this seems simple enough to catch the unhandled errors