[WIP] Fix JSON schema validation for MCP server payload

[Copilot]

Fix MCP Gateway Configuration Validation for GitHub Actions Expressions

Problem

The MCP gateway configuration validation fails with "invalid character '$' in string escape code" because GitHub Actions expressions like ${{ github.workspace }} are not being properly substituted before JSON validation.

Root Cause

The prepareConfigForValidation() function in pkg/workflow/mcp_renderer.go substitutes shell variables (e.g., $MCP_GATEWAY_PORT, ${MCP_GATEWAY_DOMAIN}) but does NOT handle GitHub Actions expressions (e.g., ${{ github.workspace }}).

Solution Plan

• Investigate current validation logic in prepareConfigForValidation()
• Add regex-based substitution for GitHub Actions expressions
• Handle all common patterns: ${{ github.* }}, ${{ env.* }}, etc.
• Test the fix with smoke-copilot workflow
• Run unit tests and validation
• Run make agent-finish before committing

Original prompt

Review validation of generated mcp server payload and fix the JSON schema or generated code. You need to take into account uninterpolated expressions:

@pelikhan ➜ /workspaces/gh-aw (main) $ ./gh-aw compile smoke-copilot --validate
⚠ Using experimental feature: safe-inputs
⚠ Generated MCP gateway configuration is not valid JSON: invalid character '$' in string escape code
⚠ Invalid workflow YAML written to: .github/workflows/smoke-copilot.invalid.yml
✗ .github/workflows/smoke-copilot.md:1:1: error: workflow schema validation failed: GitHub Actions schema validation failed: jsonschema validation failed with 'https://json.schemastore.org/github-workflow.json#'
- at '/jobs/conclusion': 'oneOf' failed, none matched
  - at '/jobs/conclusion/steps/0/uses': got null, want string
  - at '/jobs/conclusion': validation failed
    - at '/jobs/conclusion': missing property 'uses'
    - at '/jobs/conclusion': additional properties 'outputs', 'steps', 'runs-on' not allowed
- at '/jobs/detection': 'oneOf' failed, none matched
  - at '/jobs/detection/steps/0/uses': got null, want string
  - at '/jobs/detection': validation failed
    - at '/jobs/detection': missing property 'uses'
    - at '/jobs/detection': additional properties 'runs-on', 'timeout-minutes', 'outputs', 'steps' not allowed
- at '/jobs/pre_activation': 'oneOf' failed, none matched
  - at '/jobs/pre_activation/steps/0/uses': got null, want string
  - at '/jobs/pre_activation': validation failed
    - at '/jobs/pre_activation': missing property 'uses'
    - at '/jobs/pre_activation': additional properties 'runs-on', 'outputs', 'steps' not allowed
- at '/jobs/safe_outputs': 'oneOf' failed, none matched
  - at '/jobs/safe_outputs/steps/0/uses': got null, want string
  - at '/jobs/safe_outputs': validation failed
    - at '/jobs/safe_outputs': missing property 'uses'
    - at '/jobs/safe_outputs': additional properties 'env', 'outputs', 'steps', 'runs-on', 'timeout-minutes' not allowed
- at '/jobs/update_cache_memory': 'oneOf' failed, none matched
  - at '/jobs/update_cache_memory/steps/0/uses': got null, want string
  - at '/jobs/update_cache_memory': validation failed
    - at '/jobs/update_cache_memory': missing property 'uses'
    - at '/jobs/update_cache_memory': additional properties 'runs-on', 'steps' not allowed
- at '/jobs/activation': 'oneOf' failed, none matched
  - at '/jobs/activation/steps/0/uses': got null, want string
  - at '/jobs/activation': validation failed
    - at '/jobs/activation': missing property 'uses'
    - at '/jobs/activation': additional properties 'outputs', 'steps', 'runs-on' not allowed
- at '/jobs/agent': 'oneOf' failed, none matched
  - at '/jobs/agent': validation failed
    - at '/jobs/agent/steps/0/uses': got null, want string
    - at '/jobs/agent/steps/2/uses': got null, want string
  - at '/jobs/agent': validation failed
    - at '/jobs/agent': missing property 'uses'
    - at '/jobs/agent': additional properties 'runs-on', 'env', 'outputs', 'steps' not allowed

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