Add text, image and file document support for Mistral model provider

[SasinduDilshara]

Add text, file, and image document support for Ballerina Mistral model provider

Related to ballerina-platform/ballerina-library#7868

[codecov]

Codecov Report

❌ Patch coverage is 97.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.43%. Comparing base (3d77674) to head (3caa866).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
ballerina/provider_utils.bal	97.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   42.22%   50.43%   +8.20%     
==========================================
  Files           3        3              
  Lines         296      347      +51     
  Branches       86      105      +19     
==========================================
+ Hits          125      175      +50     
- Misses        171      172       +1     

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[Copilot]

Pull Request Overview

This PR adds comprehensive document support for the Ballerina Mistral AI model provider, implementing text, image, and file document capabilities. The changes align with the requirement to support multimodal interactions through the Ballerina AI library.

• Enhanced document processing to support text, image, and file documents with various content types (binary data, URLs, file IDs)
• Implemented content part generation for different document types with appropriate validation and error handling
• Added extensive test coverage for new document handling functionality including edge cases and error scenarios

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
ballerina/provider_utils.bal	Core implementation of document content generation, validation, and conversion to Mistral-specific content parts
ballerina/tests/tests.bal	Comprehensive test cases for text, image, and file document support with various content types and error scenarios
ballerina/tests/test_values.bal	Updated test data structures and expected values to support new multimodal content testing
ballerina/tests/test_utils.bal	Modified utility functions to handle content parts instead of plain strings and added mock responses for document tests
ballerina/tests/test_services.bal	Updated test service to validate content parts structure instead of plain string content
ballerina/Dependencies.toml	Updated dependencies to include constraint validation and array utilities

Comments suppressed due to low confidence (2)

ballerina/tests/tests.bal:227

• The test message is misleading since pdf5 contains an invalid URL, not pdf. The error message should match the actual test case being validated.

        test:assertFail("Expected an error for invalid URL in the file document.");

ballerina/tests/tests.bal:239

• The test message mentions 'invalid URL' but this test case is actually validating pdf which contains a FileId, not an invalid URL. The error message should accurately reflect the test scenario.

        test:assertFail("Expected an error for invalid URL in the file document.");