feat: add support for message batching api

[StevanFreeborn]

Closes #16

• feat: implement CreateMessageBatchAsync method
• fix: use proper count tokens endpoint
• fix: make correct properties nullable
• tests: add integration tests for remaining 🥲 and 😀 paths
• tests: add end-to-end test for creating a message batch
• feat: add class for representing batch statuses
• feat: implement GetMessageBatchAsync method
• fix: default nullable properties to actually datetimeoffset min value when new'd up
• feat: implement GetMessageBatchResultsAsync method
• refactor: move local function below return
• tests: add test for batch result type and succeeded batch result model
• tests: cover null responses
• feat: implement ListMessageBatchesAsync method
• feat: add ListAllMessageBatchesAsync method and corresponding tests
• feat: add CancelMessageBatchAsync method and corresponding tests
• tests: add end to end test for canceling a batch
• refactor: add private method to dry class up a bit
• feat: add DeleteMessageBatchAsync method and MessageBatchDeleteResponse model with tests
• docs: stub out batch documentation
• docs: add examples for message batch api methods

[codecov]

Codecov Report

Attention: Patch coverage is 98.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 98.98%. Comparing base (0b81bfc) to head (fc2932f).
Report is 29 commits behind head on main.

Files with missing lines	Patch %	Lines
src/AnthropicClient/AnthropicApiClient.cs	96.10%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
+ Coverage   98.23%   98.98%   +0.75%     
==========================================
  Files          70       81      +11     
  Lines        1356     1478     +122     
  Branches      119      126       +7     
==========================================
+ Hits         1332     1463     +131     
  Misses          5        5              
+ Partials       19       10       -9     

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