feat: add files api support

[StevanFreeborn]

closes #35

• feat: initial implementation of creating a file via the Files API
• fix: remove unnecessary usings
• tests: add create file request tests
• docs: fix xml comments
• tests: add integration test for creating file
• tests: add end to end test
• docs: add note about files beta status
• tests: add tests for anthropic file model
• feat: implement listing a page of files
• feat: implement list all files method
• feat: implement GetFileInfoAsync, GetFileAsync, and DeleteFileAsync
• tests: add tests for sad path in new files methods
• refactor: remove unnecessary if checks and add test to handle getting null file

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.01%. Comparing base (e351823) to head (8d2b023).
Report is 25 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
+ Coverage   98.94%   99.01%   +0.06%     
==========================================
  Files          93       98       +5     
  Lines        1619     1733     +114     
  Branches      137      143       +6     
==========================================
+ Hits         1602     1716     +114     
  Misses          5        5              
  Partials       12       12              

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[Copilot]

Pull Request Overview

Adds beta support for the Anthropic Files API, including client implementation, models, converters, tests, and documentation.

• Introduces new file-related models and updates JSON source converter
• Implements CreateFileAsync, ListFilesAsync, ListAllFilesAsync, GetFileInfoAsync, GetFileAsync, and DeleteFileAsync (with multipart upload support)
• Provides unit, integration, and end-to-end tests for the new functionality and updates the README with usage examples

Reviewed Changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/AnthropicClient.Tests/Unit/Models/UrlSourceTests.cs	Adds serialization and constructor tests for UrlSource
tests/AnthropicClient.Tests/Unit/Models/ImageContentTests.cs	Expands unit tests for ImageContent constructors
src/AnthropicClient/Models/UrlSource.cs	Implements UrlSource model
src/AnthropicClient/Models/CreateFileRequest.cs	Introduces CreateFileRequest with validation
src/AnthropicClient/AnthropicApiClient.cs	Adds file endpoints and SendFileRequestAsync logic
README.md	Documents Files API usage and beta header requirement

Comments suppressed due to low confidence (2)

tests/AnthropicClient.Tests/Unit/Models/ImageContentTests.cs:134

• [nitpick] The test name says 'NullException' but asserts ArgumentNullException. Rename the method to ItShouldThrowArgumentNullException for clarity and consistency with other tests.

  public void Constructor_WhenCacheControlIsNull_ItShouldThrowNullException()

src/AnthropicClient/Models/UrlSource.cs:16

• [nitpick] Constructors don't have return values; remove the <returns> tag and move any descriptive text into the <summary> or a <remarks> section to adhere to XML comment conventions.

  /// <returns>A new instance of <see cref="UrlSource"/> with the type set to "url".</returns>