fix(frost/signing): gate RFC-21 Phase 1B binding with frost_native build tag (#3965)

## Summary

Forward-fix for #3866 CI: the Phase 1B binding file and test
referenced message types defined in \`//go:build frost_native\`
files but were themselves untagged. Untagged staticcheck on
the integration branch (#3866) then reported
\`undefined: nativeFROSTRoundOneCommitmentMessage\` and the
client-lint job failed.

Adds \`//go:build frost_native\` to:

- \`pkg/frost/signing/attempt_context_binding.go\`
- \`pkg/frost/signing/attempt_context_binding_test.go\`

The helpers and tests are only exercised by gated code paths
(the three message-type methods all live behind \`frost_native\`),
so the build tag is the right locus.

## Why now

PRs #3963 (Phase 1A) and #3964 (Phase 1B) were merged into the
\`feat/frost-schnorr-migration-scaffold\` branch before #3866's
integration CI ran. Once the merges landed, #3866's
\`client-lint\` job rebuilt under the untagged staticcheck pass
and exposed the missing tag. This PR is the smallest possible
fix.

## Verification

Locally with module-pinned staticcheck 2025.1.1:

\`\`\`
go build ./...
go build -tags 'frost_native frost_tbtc_signer' ./pkg/frost/...
go test  -tags 'frost_native frost_tbtc_signer' ./pkg/frost/signing/
staticcheck -checks \"-SA1019\" ./... # whole repo, silent
staticcheck -checks \"-SA1019\" ./pkg/frost/signing  # silent
\`\`\`

## Test plan

- [ ] CI green: client-lint, client-vet, client-scan,
  client-build-test-publish all pass.
- [ ] #3866 lint job recovers once this merges into
  \`feat/frost-schnorr-migration-scaffold\`.

Author: mswilkison

pkg/frost/signing/attempt_context_binding.go

@@ -1,3 +1,5 @@
+//go:build frost_native
+
 package signing
 
 import (

pkg/frost/signing/attempt_context_binding_test.go

@@ -1,3 +1,5 @@
+//go:build frost_native
+
 package signing
 
 import (