Test/enhanced coverage

[Saboleee]

Summary

This PR adds comprehensive test coverage for intent lifecycle edge cases, pause scope enforcement, and sets up automated code coverage reporting in CI with a 70% minimum threshold.

Changes

#77 — cancel_intent unauthorized access

• Enhanced existing test to verify intent state remains unchanged after rejected unauthorized cancellation

#76 — dst_token allowlist mid-lifecycle toggle

• dst_allowlist_toggled_mid_lifecycle_does_not_retroactively_affect_open_intent: Confirms already-open intents unaffected when allowlist is enabled post-submission
• dst_allowlist_can_be_re_enabled_to_accept_previously_blocked_tokens: Verifies re-enabling allowlist restores acceptance of previously-blocked tokens

#75 — pause scope coverage

• pause_blocks_fill_intent: Confirms fill_intent blocked during pause
• pause_does_not_block_cancel_intent: Confirms cancel_intent callable while paused
• pause_blocks_submit_accept_fill_but_allows_cancel_and_slash: Comprehensive scope verification

#78 — Code coverage CI integration

• Added cargo-tarpaulin-based coverage job with 70% minimum threshold
• Integrated Codecov for coverage tracking
• Added codecov badge to README

Test coverage

• 7 new test functions covering allowlist lifecycle, pause scope, and authorization
• All tests follow existing patterns and naming conventions
• CI job added without breaking existing workflow

Closes #76, Closes #77, Closes #75, Closes #78

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