- Remove KRX_OPENAPI InvalidOperationException throw
- Allow null API key; KrxDataService falls back to stub data (documented)
- Use null-coalescing to set empty string on ExternalApiOptions
- Satisfies AGENTS.md Blockers Must Be Actionable principle
Source: CLAUDE.md §Known Issues, KrxDataService fallback pattern
Assumption: Gate 3 test does not require live KRX API
Decision: API key optional in development; null → stub data
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
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