feat: Add shapes.txt validator

[cswilson252]

Summary:

Add validator that checks if either a shapes.txt and/or a fixed or zone-based DRT service is present (as per #1792)

DRT functionality added as per this reference

Fixes #1792

Expected behavior:

Validator will print WARNING if shapes.txt and signs of a DRT feature are not found.

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• Run the unit tests with gradle test to make sure you didn't break anything
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[Copilot]

Pull request overview

Adds a new validator intended to warn when shapes.txt is missing unless the feed appears to use zone-based or fixed-stop DRT (per #1792).

Changes:

• Introduces MissingShapesFileValidator and a corresponding test class.
• Adds unit tests covering shapes present + DRT present scenarios (and a no-shapes/no-DRT scenario).
• Adds an extra import in ShapeUsageValidator.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 9 comments.

File	Description
main/src/main/java/org/mobilitydata/gtfsvalidator/validator/MissingShapesFileValidator.java	New multi-file validator for warning on missing shapes.txt unless DRT indicators are present.
main/src/test/java/org/mobilitydata/gtfsvalidator/validator/MissingShapesFileValidatorTest.java	New unit tests for the validator behavior.
main/src/main/java/org/mobilitydata/gtfsvalidator/validator/ShapeUsageValidator.java	Adds an import for a nested notice type.

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