feat: header passthrough

[seefs001]

请求头透传
使用方式 :
{
anthropic-version: {client_header:anthropic-version}
}

先在后端进行更改，后续会采用 #2365 的部分前端变更作为UI编辑页

Summary by CodeRabbit

• New Features

  • Header override values now support dynamic placeholders (API key and incoming request header references) and are resolved at request time for more flexible header customization.

• Bug Fixes / Reliability

  • Placeholder processing now validates syntax, skips empty replacements to avoid injecting empty headers, and surfaces clearer errors for invalid templates.

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[coderabbitai]

Walkthrough

Adds runtime placeholder resolution for header overrides ({api_key}, {client_header:}) via a new helper. processHeaderOverride now accepts a *gin.Context; DoApiRequest, DoFormRequest, and DoWssRequest pass the context so client request headers can be used during override resolution.

Changes

Cohort / File(s)

Summary

Header override processing relay/channel/api_request.go

Added clientHeaderPlaceholderPrefix constant and applyHeaderOverridePlaceholders(template string, c *gin.Context, apiKey string) (string, bool, error). Modified processHeaderOverride(info *common.RelayInfo, c *gin.Context) to use the helper and treat empty results as omitted. Updated DoApiRequest, DoFormRequest, and DoWssRequest to pass gin.Context into processHeaderOverride. Errors from placeholder processing are wrapped as ChannelHeaderOverrideInvalid.

Sequence Diagram

sequenceDiagram
    participant Client
    participant Handler as API Handler (DoApiRequest / DoFormRequest / DoWssRequest)
    participant Processor as processHeaderOverride
    participant Resolver as applyHeaderOverridePlaceholders
    participant OutReq as Outgoing Request

    Client->>Handler: Incoming request + gin.Context
    Handler->>Processor: processHeaderOverride(info, gin.Context)
    Processor->>Resolver: applyHeaderOverridePlaceholders(template, gin.Context, apiKey)
    
    rect rgba(100, 150, 200, 0.5)
        Note over Resolver: Parse {api_key} and {client_header:<name>}<br/>Validate syntax & context, then substitute runtime values
    end

    Resolver-->>Processor: resolvedValue + includeFlag
    alt includeFlag == true
        Processor->>OutReq: add header with resolvedValue
    else
        Processor->>OutReq: omit header
    end

    OutReq-->>Client: response

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Estimated Code Review Effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly Related PRs

• feat: enhance Authorization header handling with Header Override support #2738: Modifies header-override handling in the same area and reorders header setup vs override application — closely related to context-passing change.
• feats:add custom headers override #1644: Introduced header-override plumbing and error codes that this change builds upon.

Poem

🐰 I hopped into code with a curious cheer,
Found {api_key} and client headers near,
I stitched them at runtime with gin in my paw,
Now outgoing requests carry what they saw,
Hop, hop—placeholders resolved, hooray! 🎉

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 16.67% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'feat: header passthrough' is directly related to the changeset, which implements header passthrough functionality with placeholder processing for client headers and API keys.

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✨ Finishing touches

• 📝 Generate docstrings

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@relay/channel/api_request.go`:
- Around line 41-73: The applyHeaderOverridePlaceholders function currently
performs {api_key} interpolation after handling a {client_header:...}
placeholder, allowing a client-supplied header to include "{api_key}" and leak
the key; modify applyHeaderOverridePlaceholders so that when a client header
placeholder is detected (strings.HasPrefix(trimmed,
clientHeaderPlaceholderPrefix)) and a non-empty clientHeaderValue is obtained,
the function returns immediately with that value (return template, true, nil)
instead of continuing to the {api_key} replacement logic; update the branch that
sets template = clientHeaderValue in applyHeaderOverridePlaceholders to return
early and keep existing error/empty-value behavior unchanged.