[Experimental] HTTP agent runtime

[lspinheiro]

Why are these changes needed?

This is an initial proposal for an experimental http agent runtime based on fastapi and json-rpc. The goal is to support existing middleware system to facilitate enhancement of agent functionality such as enabling authentication.

Related issue number

Related to #6355

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

Attention: Patch coverage is 61.65884% with 245 lines in your changes missing coverage. Please review.

Project coverage is 78.85%. Comparing base (cc2693b) to head (8ab2a22).

Files with missing lines	Patch %	Lines
...t/src/autogen_ext/runtimes/http/_worker_runtime.py	63.55%	129 Missing ⚠️
...ext/runtimes/http/_worker_runtime_host_servicer.py	53.50%	73 Missing ⚠️
.../autogen_ext/runtimes/http/_worker_runtime_host.py	60.90%	43 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6356      +/-   ##
==========================================
- Coverage   79.52%   78.85%   -0.67%     
==========================================
  Files         225      230       +5     
  Lines       16641    17280     +639     
==========================================
+ Hits        13233    13627     +394     
- Misses       3408     3653     +245     

Flag	Coverage Δ
unittests	78.85% <61.65%> (-0.67%)	⬇️

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[yogitasrivastava]

1. Content-type
   Consider enforcing a stricter Content‑Type: application/json on incoming requests early, with clear 415/400 responses for bad payloads.

2. Runtime vs. Host separation
   The split between HttpWorkerAgentRuntime and HttpWorkerAgentRuntimeHost is clean, but the naming feels a bit opaque (the “HostServicer” suffix sounds gRPC‑ish). Kindly consider renaming the host side to something like HttpAgentServer or HttpAgentHost for clarity.