Fix: Prevent CORS exposure and sensitive error disclosure

[mertcano]

This PR mitigates critical data disclosure and unauthorized origin execution vectors in the FastAPI RPC server.

Updates:

CORS Hardening: Removed the permissive allow_origins=["*"] configuration. The server now requires a comma-separated ALLOWED_ORIGINS environment variable and will strictly fail closed (RuntimeError) on startup if it is omitted. This ensures a deliberate deployment policy is always enforced.

Error Disclosure Prevention: Removed the str(exc) reflection in the /api endpoint's fallback exception handler. Provider, RPC, and internal contract state anomalies are now safely logged to the server backend while the client receives a static An unexpected server error occurred response detail.

Fixes #issue-number-here

What

• changed thing a for b
• also did this other unrelated thing in my path

Why

• to fix a bug
• to add more value to the user

Testing done

• tested the new feature
• tested the bug fix

Decisions made

Checks

• I have tested this code
• I have reviewed my own PR
• I have created an issue for this PR
• I have set a descriptive PR title compliant with conventional commits

Reviewing tips

User facing release notes

[github-actions]

This PR targeted main, which is only the default/static branch.

I retargeted it to v0.123-dev, the active development branch. Pushes to v0.123-dev automatically fast-forward main.

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