Add unit tests for the typed configuration validation layer

[Lakes41]

• Difficulty: Intermediate
• Type: test
• Background: The README describes a "typed configuration layer" validating EXPO_PUBLIC_API_URL and EXPO_PUBLIC_CHAIN_ID at startup, throwing a developer-facing error if missing/invalid rather than silently falling back to production defaults.
• Problem: This safety-critical startup validation logic has no documented/visible test coverage, risking silent regressions that reintroduce unsafe fallbacks.
• Expected outcome: A dedicated test file verifying: missing var throws, malformed URL throws, invalid chainId (non-numeric or unsupported) throws, and valid config passes through unchanged.
• Suggested implementation: Locate the config module (likely src/config/ or src/lib/config.ts), write Vitest cases mocking process.env/Constants.expoConfig.extra, assert thrown error messages are actionable.
• Acceptance criteria: All four scenarios above are covered; tests fail if fallback-to-production-default behavior is reintroduced; pnpm typecheck remains clean.
• Likely affected files/directories: src/ (config module), tests/
• Labels: test, good first issue, GrantFox OSS, Maybe Rewarded, Official Campaign | FWC26

[web3nova]

Hello @maintainer, I’ve gone through the requirements and I’m confident I can implement a proper solution that matches the acceptance criteria. Kindly assign this issue to me.

[grantfox-oss]

🦊 GrantFox — @web3nova has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #77)
2. Your PR will be reviewed by the Adamantine-guild maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@web3nova's PR #276 was approved and merged by @Lakes41.

🏆 @web3nova: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (236 total points). Track your full progress on GrantFox.

👏 Great work, @web3nova! Keep contributing to Adamantine-guild.