[IPF-87-1] DKIM record validation updated in ParsePublicKey().

[illyasch]

This PR improves DKIM record parsing and validation and enhances error handling.

1. New Error Definitions

The patch introduces explicit error constants to improve clarity and debugging:

• ErrEmptyRecord: Indicates an empty DKIM record.
• ErrUnsupportedVersion: Indicates an unsupported DKIM record version.
• ErrUnsupportedAlgorithm: Indicates an unsupported algorithm in DKIM.
• ErrDecodeBase64: Indicates errors during base64 decoding.
• ErrParsePublicKey: Indicates failures during public key parsing.
• ErrUnsupportedServices: Indicates unsupported service types listed in the DKIM key.
• ErrMissedPTag: Indicates the absence of the mandatory p tag.

2. Improvements in ParsePublicKey

• Multiple Error Reporting:
  The function now collects and returns multiple errors encountered during parsing instead of halting after the first error.

• Updated Function Signature:
  ParsePublicKey returns a slice of errors ([]error) instead of a single error.

[Copilot]

Pull Request Overview

This PR improves DKIM record parsing and validation by introducing explicit error constants and enabling the collection of multiple errors during parsing. Key changes include:

• Adding new error constants and updating error messages.
• Refactoring ParsePublicKey to return a slice of errors, accumulating all validation issues.
• Adjusting tests and cache logic to account for the updated error handling and removing obsolete tool dependency code.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

File	Description
dkim.go	Updated constants, ParsePublicKey and validatePublicKey to support multiple error reporting.
dkim_test.go	Updated tests to reflect the new multi-error handling and error joining logic.
internal/tooldeps/toolsdeps.go	Removed unused tool dependency file.

Comments suppressed due to low confidence (1)

dkim.go:726

• [nitpick] The variable 'required' is used to track if the mandatory 'p' tag is present. For improved clarity, consider renaming it to 'pTagFound' or a similar descriptive name.

var errs []error
	required := false