Support environment variables in RewritePlugin replacement values

[waldekmastykarz]

Adds support for referencing environment variables in RewritePlugin replacement values using ${env:VAR_NAME}.

Changes

• out.url and out.headers[].value can contain ${env:VAR_NAME} tokens.
• Values are inserted as-is, so in out.url they can reference capture groups from the in.url pattern.
• Variables are resolved only for requests matching in.url. in.url itself isn't expanded.
• If a referenced variable isn't set, the plugin logs an error and skips the rule (URL and headers).
• Updated v4.0.0 rewrites file schema descriptions and skill references.
• Added integration tests for expansion and missing-variable handling.

Example

{
  "rewrites": [
    {
      "in": { "url": "https://api.contoso.com/(.*)" },
      "out": {
        "url": "${env:API_BASE_URL}/$1",
        "headers": [ { "name": "Authorization", "value": "Bearer ${env:API_TOKEN}" } ]
      }
    }
  ]
}

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation matches the documented behavior and includes appropriate integration coverage.

Review effort: Balanced
Findings: None

What changed in this PR

Adds environment-variable expansion to RewritePlugin output values while safely skipping rules with missing variables.

Changes:

• Expands ${env:VAR_NAME} in rewritten URLs and headers.
• Adds integration coverage for expansion and missing variables.
• Updates schema and skill documentation.

File	Description
RewritePlugin.cs	Implements environment-variable expansion.
ManipulationAndMockingIntegrationTests.cs	Tests expansion and skipped rules.
rewriteplugin.rewritesfile.schema.json	Documents supported tokens.
plugin-catalog.md	Updates plugin reference.
configuration.md	Updates configuration guidance.

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