musig: Reject partial sig without generated secnonce #35419

Summary

• Replace MuSig2SecNonce::IsValid() with HasNonce(), set only after a successful CreateMuSig2Nonce and cleared by Invalidate().
• Reject CreateMuSig2PartialSig when the secnonce was never generated, and guard the signing provider path in MutableTransactionSignatureCreator::CreateMuSig2PartialSig.
• Document the intended signing API on MuSig2SecNonce and add a unit test.

Previously IsValid() only checked that secure memory was allocated, not that nonce generation succeeded. A caller could reach partial signing with an uninitialized secnonce.

Test plan

• build/bin/test_bitcoin --run_test=bip328_tests
• build/bin/test_bitcoin --run_test=psbt_tests (unchanged behavior, sanity)
• Manual MuSig2 PSBT signing flow still works end-to-end

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35419.

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