Update Anthropic kwargs + docs

[svilupp]

Added

• Added beta headers to enable long outputs (up to 8K tokens) with Anthropic's Sonnet 3.5 (see ?anthropic_extra_headers).
• Added a kwarg to prefill (aiprefill) AI responses with Anthropic's models to improve steerability (see ?aigenerate).

Updated

• Documentation of aigenerate to make it clear that if streamcallback is provide WITH flavor set, there is no automatic configuration and the user must provide the correct api_kwargs.
• Grouped Anthropic's beta headers as a comma-separated string as per the latest API specification.

[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.85%. Comparing base (5c198fb) to head (43f4a81).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/streaming.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #204      +/-   ##
==========================================
+ Coverage   91.83%   91.85%   +0.02%     
==========================================
  Files          46       46              
  Lines        4359     4372      +13     
==========================================
+ Hits         4003     4016      +13     
  Misses        356      356              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.