test: expand coverage for core utilities, providers, and error handling

[devhimanshuu]

🎯 What does this PR do?

This PR significantly improves the stability of Passmark by expanding test coverage for core components that were previously under-tested.

It successfully brings line coverage to 100% for:

• src/errors.ts
• src/email.ts
• src/providers/emailsink.ts

✨ Key Changes

• Custom Errors (src/errors.ts): Added comprehensive unit tests for all custom error classes to ensure they instantiate correctly with the appropriate machine-readable error codes.
• Email Sink Provider (src/providers/emailsink.ts): Implemented isolated tests mocking the fetch API. This verifies successful content extraction, handles JSON parsing correctly, and properly catches edge-case errors.
• Email Extraction (src/email.ts): Added tests for extractEmailContent using Vitest's fake timers. This allowed us to verify retry logic, timeouts, and exponential backoff behavior without slowing down the overall test suite.

[Copilot]

Pull request overview

This PR expands Vitest coverage for Passmark’s core utilities around email handling, provider integration, and the custom error hierarchy to improve confidence and regression protection.

Changes:

• Adds unit tests for PassmarkError and all custom error subclasses (verifying names/codes/properties).
• Adds isolated tests for the Emailsink provider, including URL construction, JSON parsing behavior, and error cases.
• Adds tests for extractEmailContent covering initial configuration failure, success, retry behavior, and max-retry failure using fake timers.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/tests/providers/emailsink.test.ts	New tests for Emailsink provider fetch/URL behavior, parsing, and failure cases.
src/tests/errors.test.ts	New tests validating custom error classes and their machine-readable codes.
src/tests/email.test.ts	Extends email tests to cover extractEmailContent retry/timeout flow using fake timers.

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