feat: Add a custom header to the SDK for chatting with the agent.

[LeiZhang-Hunter]

What problem does this PR solve?

As title.

Type of change

• New Feature (non-breaking change which adds functionality)

[asiroliu]

@LeiZhang-Hunter

1. I checked your PR—the changes are actually for the Web API, not the SDK.
2. I'm concerned that custom headers could open us up to HTTP Header Injection and parsing risks, which aren't being handled in the current code.

[LeiZhang-Hunter]

• I checked your PR—the changes are actually for the Web API, not the SDK.

• I'm concerned that custom headers could open us up to HTTP Header Injection and parsing risks, which aren't being handled in the current code.

ragflow only doing pass-through; you don't need to worry about these things.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.46%. Comparing base (2e5a186) to head (1eff498).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12430   +/-   ##
=======================================
  Coverage   44.46%   44.46%           
=======================================
  Files          43       43           
  Lines        9266     9266           
  Branches      107      107           
=======================================
  Hits         4120     4120           
  Misses       5127     5127           
  Partials       19       19           

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