Add Files API endpoints

[Flavinou]

Added Files API endpoints based on documentation (https://docs.databricks.com/api/azure/workspace/files)

Tests

• Added serialization/ deserialization unit tests
• Added sample code + tested in real workspace environment

Resolves #235

[Flavinou]

@microsoft-github-policy-service agree company="Ubisoft"

[memoryz]

Hi @Flavinou , appreciate the PR. However, could you please remove all the changes that include only whitespace and format changes? Your PR has 215 changed files, and it's super time consuming for me to walk through them one by one.

[Flavinou]

Sure ! Sorry about that, I blindly followed the contribution guidelines when I should have done it in another PR.

[Copilot]

Pull Request Overview

Adds support for Databricks Files API by introducing client-side models, interface, implementation, tests, and sample usage

• Introduce DirectoryEntry and DirectoriesList models to represent file system entries and pagination
• Define IFilesApi and implement it in FilesApiClient, extending ApiClient with HEAD support and wiring into DatabricksClient
• Add comprehensive unit tests and sample program scenarios for all file and directory operations

Reviewed Changes

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

Show a summary per file

File	Description
Models/DirectoryEntry.cs	New record for file/directory metadata
Models/DirectoriesList.cs	New paginated directory listing model
IFilesApi.cs	Interface defining Files API methods
FilesApiClient.cs	Implements file and directory operations
ApiClient.cs	Refactored to add HEAD support (HttpHead) and shared response helper
DatabricksClient.cs	Exposes Files API on the root client and disposes it
FilesApiClientTest.cs	Unit tests covering directory and file operations
SampleProgram.cs / SampleProgram.Files.cs	Sample scenarios demonstrating Files API usage

Comments suppressed due to low confidence (2)

csharp/Microsoft.Azure.Databricks.Client/FilesApiClient.cs:31

• Adding tests for ListDirectoryContents with non-default pageSize and pageToken would catch URL-building bugs and ensure pagination logic is correct.

        if (pageSize != null)

csharp/Microsoft.Azure.Databricks.Client/Models/DirectoryEntry.cs:30

• The API returns last_modified as milliseconds since Unix epoch. Ensure you have a custom JSON converter registered (e.g. a JsonConverter for epoch milliseconds) otherwise deserialization to DateTimeOffset will fail or produce incorrect values.

    public DateTimeOffset? LastModified { get; set; }

[memoryz]

@Flavinou I added a commit to your PR to add support for pageable list and also addressed some of the Copilot code review comments. Could you please take a look and kindly test the ListDirectoryContentsPageable and Download functions?

[Flavinou]

@memoryz Of course, thank you for the follow up I didn't know about the Azure built-in pageable features !
I will test it shortly

[Flavinou]

Tested with a real workspace, it looks fine to me - I edited the sample program to list the directory contents based on the user input, it's minor