fix: reject malformed mcp header templates

[HwangJohn]

Summary

• parse MCP HTTP header values containing ${...} as env templates
• reject malformed, invalid, missing, or unresolved template fragments instead of sending them literally
• keep valid env-backed header values and literal secret rejection behavior working

Why

MCP connector headers are allowed to reference credentials through env templates. Malformed templates like Bearer ${TOKEN should fail closed rather than being passed through as literal header values.

No dedicated issue; found during MCP connector hardening.

Testing

• corepack pnpm exec vitest run test/mcp-client.test.ts passed
• corepack pnpm run format:check passed
• corepack pnpm run lint:check passed
• corepack pnpm run typecheck passed
• git diff --check passed
• corepack pnpm test was attempted on Windows and still fails outside this change in existing test/env-behavior.test.ts HOME/permission assertions plus timeout-prone Mermaid/jsdom tests