Fix HMAC JWK import length handling

[harrshita123]

Fixes #302.

What changed

• Pass length through HMAC JWK import.
• Apply length after decoding jwk.k, matching Web Crypto HMAC import steps.
• Keep raw and JWK import behavior consistent for non-byte-aligned lengths.
• Add regression coverage for raw/JWK parity and invalid HMAC key length.

Testing

• /Users/apple/flutter/bin/dart run lib/src/testing/regression/issue_302_hmac_jwk_length.dart
• /Users/apple/flutter/bin/dart test test/webcrypto_test.dart
• /Users/apple/flutter/bin/dart analyze

[harrshita123]

Sorry, I accidentally clicked the close button. I have reopened the PR.

[harrshita123]

@jonasfj I resolved all conflicts.

[HamdaanAliQuatil]

I checked this against the normative HMAC import steps and left the details in #302. Unlike hash or curve, length is applied after the JWK has been decoded: the imported key consists of the first length bits.

That means the current validation-only implementation, particularly the test rejecting non-zero discarded bits, would encode behavior that differs from Web Crypto. I think we should settle the expected behavior in #302 before merging this.

[harrshita123]

Thanks for the clarification and for checking the HMAC import steps against the Web Crypto spec.

I updated the PR so JWK import now follows the same behavior as raw import: it decodes jwk.k first, then applies length by using the first length bits. I also removed the validation-only behavior that rejected non-zero discarded bits, since that would not match the spec.

The regression test now covers the raw/JWK parity case for { k: "_-A" } with length: 9, and the focused regression test, dart analyze, and test/webcrypto_test.dart are passing.