feat: add :req_plugins option to high-level APIs

[mikehostetler]

Problem

Users need to apply Req plugins (logging, caching, middleware) without dropping to low-level API.

Solution

Added :req_plugins option that accepts a list of (Req.Request.t() -> Req.Request.t()) functions.

Supported Functions

• ReqLLM.generate_text/3
• ReqLLM.generate_text!/3
• ReqLLM.generate_object/4
• ReqLLM.generate_object!/4
• ReqLLM.embed/3
• ReqLLM.Images.generate_image/3

Note: Streaming functions (stream_text/3, stream_object/4) use Finch directly and do not support :req_plugins.

Usage

# Add custom header
ReqLLM.generate_text(model, prompt,
  req_plugins: [
    fn req -> Req.Request.put_header(req, "X-Custom", "value") end
  ]
)

# Chain multiple plugins (executed in order)
ReqLLM.generate_text(model, prompt,
  req_plugins: [
    &MyApp.ReqPlugins.add_logging/1,
    &MyApp.ReqPlugins.add_tracing/1
  ]
)

Testing

• Added 3 tests verifying plugins are applied and order is preserved

Closes #359