rpcserver: add header extraction and transformation helpers

[odysseus0]

Summary

Add utilities for extracting and transforming HTTP headers in the JSON-RPC request chain.

New: headers.go

• ExtractHeaders(req, names) - extract specified headers from request
• TransformToFlashbots(headers) - transform User-Agent→X-Flashbots-User-Agent, Origin→X-Flashbots-Http-Origin
• Header constants: HeaderFlashbotsUserAgent, HeaderFlashbotsHttpOrigin, HeaderFlashbotsOrigin, etc.
• Default lists: EdgeHeaders, FlashbotsHeaders

Modified: jsonrpc_server.go

• Added Header(ctx, name) - nil-safe helper to get header value from request in context

Usage

// Edge service (protect-rpc): extract and transform
edge := rpcserver.ExtractHeaders(req, rpcserver.EdgeHeaders)
transformed := rpcserver.TransformToFlashbots(edge)

// Passthrough service (protect-of-api): forward existing
existing := rpcserver.ExtractHeaders(req, rpcserver.FlashbotsHeaders)

// Storage service (mev-share-node): read from context
userAgent := rpcserver.Header(ctx, rpcserver.HeaderFlashbotsUserAgent)
httpOrigin := rpcserver.Header(ctx, rpcserver.HeaderFlashbotsHttpOrigin)

Test plan

• Unit tests for ExtractHeaders, TransformToFlashbots
• Tests for nil request handling
• Tests for header constants

🤖 Generated with Claude Code