Make login, challenge, and session issuance resilient in `src/routes/auth.ts`

[Jagadeeshftw]

Description

The current path can fail in ways that are hard to retry or safely replay. src/routes/auth.ts owns login, challenge, and session issuance, but the current behavior does not yet define the contract tightly enough for safe maintenance. Design the change so the runtime path, tests, and docs all describe the same behavior.

Requirements and context

• Keep the contract specific to src/routes/auth.ts and its existing callers.
• Add coverage for the success path and the failure or boundary path relevant to this issue.
• Preserve compatibility where older callers or existing tests already rely on the current shape.
• Keep the change easy to review and small enough for one focused PR.

Suggested execution

Fork the repo and create a branch

git checkout -b auth-route-reliability-05

Implement changes

• Update/Write: src/routes/auth.ts
• Write comprehensive tests: src/routes/auth.test.ts
• Add documentation: docs/routes/auth.md
• Keep any new helper names consistent with the rest of src/

Test and commit

• Run pnpm test
• Run pnpm lint
• Add a note for any edge cases that are intentionally out of scope

Example commit message

fix: improve reliability and retry semantics

Guidelines

• WCAG 2.1 AA is not relevant here; keep the focus on correctness and maintainability.
• Prefer a narrow, well-documented fix over a broad refactor.
• Timeframe: 96 hours

[Stellahub-64]

@Stellahub-64 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

i will love to work on this issue

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[Jagadeeshftw]

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This issue has been completed by @Stellahub-64 as part of the Stellar Wave Program's 7th Wave 🥳

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