test: add NA-handling test for mixed date formats with explicit format

[tanmaydimriGSOC]

Summary

This PR adds a focused, high-value test to ensure that parse_date() behaves correctly when given mixed date formats while an explicit format is supplied. The test verifies that:

• Strings matching the format are parsed successfully.
• Strings not matching the format are returned as NA.
• A warning is issued for mismatched formats.

This scenario was not previously covered in the test suite and improves coverage for real-world edge cases.

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Rationale

When users provide an explicit format, parse_date() should consistently:

1. Parse valid inputs matching the format.
2. Fail gracefully with NA for mismatched or invalid inputs.
3. Emit a clear, informative warning.

Adding this test ensures that these expectations remain stable and prevents future regressions around mixed-format date parsing — a common source of issues when working with heterogeneous datasets.

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What this test covers

The added test checks a small vector containing:

• A valid date matching the supplied format
• A date in a different format
• A completely invalid date

It then asserts that:

• Matching entries parse correctly
• Non-matching entries return NA
• A warning is produced

The test fits neatly within the existing structure of test-parsing-datetime.R.

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Files changed

• tests/testthat/test-parsing-datetime.R (new test block only)

No source code or snapshot changes.

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How to test

Run either:

testthat::test_file("tests/testthat/test-parsing-datetime.R")

or

devtools::test()

Both will execute the new unit test.

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This PR strengthens the reliability of date parsing in readr and adds coverage for a meaningful edge case that was previously untested.