Clarify regex match error in pytest.raises

[dariomesic]

Purpose

This PR addresses a suggestion in AbstractRaises._check_match (inside src/_pytest/raises.py) which noted that the error message for a regex mismatch could be confusing.

The previous message used Regex: and Input:, which could be ambiguous in failure logs.

This change updates the _fail_reason string to be more explicit:

• Regex: -> Expected regex:
• Input: -> Actual message:

This makes the failure message clearer and easier to debug, as requested in the code comment.

Tests

This change required updating the assertions in several tests that were checking for the exact old error message. The following test files were updated to expect the new, clearer message:

• testing/python/raises.py
• testing/python/raises_group.py

[bluetech]

Thanks!