test: add final QA smoke suite

[saurabhhhcodes]

Summary

• add a deterministic final QA smoke suite for budget state, router validation, fallback routing, provider metadata, and request/response transforms
• add npm test scripts so maintainers can run build plus QA and cross-provider suites in one command
• document real-API manual QA steps for prompt types, analytics accuracy, and error handling

Validation

• npm test
• git diff --check

Closes #38

[saurabhhhcodes]

Quick review/scoring follow-up: this PR is clean from my side and has no checks configured in the repo.

Validation already run:

• npm test
• git diff --check

Could you please review when you get a chance and add the appropriate GSSoC labels before merge? The linked issue is #38 and the PR currently has no visible scoring labels.