Remove sync over async, convert APIs to async

[kevingosse]

No description provided.

[Copilot]

Pull request overview

This PR converts the Braintrust SDK from synchronous to asynchronous APIs, eliminating the sync-over-async anti-pattern and improving scalability. The changes include converting all API client methods to return Task<T>, updating the Eval framework to support async build and run operations, and migrating all tests and examples to use async/await patterns.

Changes:

• Converted IBraintrustApiClient interface and BraintrustApiClient implementation to async methods, removing sync-over-async patterns
• Refactored Eval<TInput, TOutput> to use async initialization in BuildAsync() and async execution in RunAsync()
• Extracted ApiException to a separate file and moved MockBraintrustApiClient to its own test file for better organization

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Braintrust.Sdk/Api/IBraintrustApiClient.cs	Converted all method signatures to return Task for async operations
src/Braintrust.Sdk/Api/BraintrustApiClient.cs	Removed sync-over-async anti-pattern, converted methods to proper async/await with ConfigureAwait(false)
src/Braintrust.Sdk/Api/ApiException.cs	Extracted ApiException class to separate file for better organization
src/Braintrust.Sdk/Eval/Eval.cs	Converted Build() to BuildAsync() and Run() to RunAsync(), moved async initialization from constructor to builder
src/Braintrust.Sdk/Braintrust.cs	Renamed ProjectUri() to GetProjectUriAsync() and converted to async
tests/Braintrust.Sdk.Tests/Eval/MockBraintrustApiClient.cs	Extracted mock client to separate file, converted to async methods
tests/Braintrust.Sdk.Tests/Eval/EvalTest.cs	Updated tests to async, removed mock client definition
tests/Braintrust.Sdk.Tests/Api/BraintrustApiClientTest.cs	Converted all test methods to async with proper async assertions
examples/SimpleOpenTelemetry/Program.cs	Updated Main to async and converted GetProjectUriAsync() call
examples/OpenAIInstrumentation/Program.cs	Updated GetProjectUriAsync() call
examples/EvalExample/Program.cs	Updated Main to async and converted BuildAsync()/RunAsync() calls

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[kevingosse]

Closed in favor of #10