Add request header rewriting to RewritePlugin

[waldekmastykarz]

Adds the ability to set request header values with RewritePlugin.

Changes

• Rewrite rules support out.headers ([{ "name", "value" }]).
• out.url is now optional, so a rule can match on in.url and only change headers.
• Header names are matched case-insensitively. An existing header's value is replaced; a missing header is added.
• Header rewrites apply only when in.url matches the request URL (evaluated before that rule's URL rewrite).
• Updated the v4.0.0 rewrites file schema and the skill reference docs.
• Added integration tests.

Example

{
  "rewrites": [
    {
      "in": { "url": "^https://api\\.contoso\\.com/" },
      "out": { "headers": [ { "name": "Authorization", "value": "Bearer test-token" } ] }
    }
  ]
}

[Copilot]

Copilot review overview

🟡 Changes recommended

Null header entries can abort remaining rewrite rules, and combined URL-and-header behavior needs regression coverage.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Null rewrite headers entry causes rule processing to be skipped · New
• Add regression test for header matching before URL rewriting · New

What changed in this PR

Adds request-header rewriting to Dev Proxy’s RewritePlugin alongside existing URL rewriting.

Changes:

• Supports header-only rules and case-insensitive header replacement.
• Updates the v4.0.0 schema and configuration references.
• Adds integration tests for matching and nonmatching requests.

File	Description
skills/​dev-proxy/​references/​plugin-catalog.md	Documents header rewriting.
skills/​dev-proxy/​references/​configuration.md	Describes optional rewrite outputs.
schemas/​v4.0.0/​rewriteplugin.rewritesfile.schema.json	Allows headers and optional URL output.
DevProxy.Plugins/​Manipulation/​RewritePlugin.cs	Applies matching header rewrites.
DevProxy.Integration.Tests/​ManipulationAndMockingIntegrationTests.cs	Tests header addition, replacement, and nonmatches.

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[Copilot]

Copilot review overview

🟢 Approval recommended

Only minor, nonblocking documentation and performance improvements remain; both previously reported issues are resolved.

Review effort: Balanced
Findings: None

Resolved since last review (2)

• Null rewrite headers entry causes rule processing to be skipped
• Add regression test for header matching before URL rewriting