Raise backend unit+integration coverage to 80% with a CI gate

[joelpeace48-cell]

Epic: Testing & QA (expansion)

Summary

Measure backend coverage and enforce an 80% floor, adding tests for uncovered critical paths.

Background & Problem

A coverage gate protects the 32k-LOC backend against regressions during the contributor wave.

Acceptance Criteria

• Coverage measured + gated at 80%.
• Critical paths covered.
• Report per PR.

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Filed as part of the mainnet-readiness & world-class roadmap. Scope is intentionally small enough for one focused PR; comment to claim.

[Williams-1604]

I'm relatively early in my journey with Soroban, but I've studied the codebase for Trivela and I believe I can handle this issue with care and attention to detail. I'd really appreciate the chance to prove myself here — I won't let the project down. Just like i did a perfect job last wave with no failing CI pipeline. Kindly consider my application please.