test(validation): consolidate public validation coverage

[woahwhattheheck]

Closes #1946

Summary

• keep the current single canonical validation suite at src/lib/backend/validation.test.ts
• cover the remaining public validateCommitmentId path, including the configured length boundary and unsafe identifiers
• exercise the exported Stellar public-key regex alongside the existing asset/address coverage
• repair the current-base SUPPORTED_ASSETS[].includes(...) parse defect so the validation module imports cleanly

The older duplicate test path referenced by the issue is already absent on current master; this PR completes the remaining acceptance gap against the repository as it exists now.

Scope

Two files only: the canonical validation test file and the one-character production parse repair. No API behavior redesign or broad test expansion.

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