Validate company slugs in TheirStack API and reject invalid requests

[Copilot]

Unknown company slugs were silently dropped by resolveCompanies(), causing successful-but-empty runs that were hard to debug.

Changes:

• Export ASHBY_COMPANY_SLUGS from theirstack-fetcher.ts for validation
• Add slug validation in pipeline route before execution:
  • Return 400 with invalid slug list when no valid slugs provided
  • Log warning and proceed when mix of valid/invalid slugs provided
• Validation only applies when source=theirstack and companies parameter is non-empty

Behavior:

# All invalid → 400 error
POST /api/pipeline/step1?source=theirstack&companies=badco,invalid
{
  "error": "No valid company slugs provided",
  "invalidSlugs": ["badco", "invalid"],
  "hint": "Use valid Ashby company slugs. See ASHBY_COMPANY_SLUGS for supported companies."
}

# Mixed valid/invalid → warn and proceed with valid
POST /api/pipeline/step1?source=theirstack&companies=openai,badco
# Logs: "Invalid company slugs provided: badco"
# Proceeds with: openai

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.