fix(candid): improve error messages for candid decoding errors

[yasumorishima]

Description

Improves error messages for Candid decoding errors in Variant types by providing more detailed information when a field hash cannot be found.

Changes

Enhanced error message in VariantClass.decodeValue to include:

• Received field hash and type
• Expected fields list
• Provided type definition
• Helpful hint: "Did you forget to include a variant?"

Before:

Error: Cannot find field hash _3456837_

After:

Error: Unexpected field hash _3456837_. Did you forget to include a variant?

Received field hash: _3456837_ (Record)

Expected fields: Invalid, Ok, Err
Provided type: variant {Invalid:null; Ok:Record; Err:Text}

Related Issue

Closes #470

Testing

• ✅ All existing tests pass (36 test suites, 552 tests)
• ✅ No new tests needed as this is an error message improvement
• ✅ CodeRabbit review: 0 issues
• ✅ CodeQL: 0 new alerts

Checklist

• Code follows the project's style guidelines (prettier:format passed)
• Lint checks pass (eslint)
• All tests pass
• CHANGELOG.md updated
• Follows Conventional Commits format

[cla-idx-bot]

Dear @yasumorishima,

In order to potentially merge your code in this open-source repository and therefore proceed with your contribution, we need to have your approval on DFINITY's CLA.

If you decide to agree with it, please visit this issue and read the instructions there. Once you have signed it, re-trigger the workflow on this PR to see if your code can be merged.

— The DFINITY Foundation

[Copilot]

Pull request overview

Improves developer-facing diagnostics when Candid decoding a variant fails due to an unknown/mismatched field hash, making it easier to identify missing variants or mismatched interface definitions.

Changes:

• Enhanced VariantClass.decodeValue error message to include received tag/hash info, expected variant fields, and the provided type definition.
• Updated CHANGELOG.md to record the fix under [Unreleased].

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
packages/core/src/candid/idl.ts	Expands the thrown error when a variant tag/hash can’t be matched to an expected field.
CHANGELOG.md	Adds an Unreleased entry describing the improved variant decoding error message.

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[nikosxenakis]

Overall looks good, minor comment to address.

[nikosxenakis]

Thank you @yasumorishima for the contribution, I am merging it.