fix: return 400 on invalid Stellar address in backend routes

[ryzen-xp]

Description

Several backend routes accept a Stellar public key as a path or query param but do not validate its format, leading to confusing on-chain errors. Add format validation and return HTTP 400 early when the address is invalid.

Requirements

• Validate Stellar address format (starts with G, 56 chars) in affected routes
• Return 400 with a descriptive message on invalid input
• Add tests for valid and invalid address inputs

Acceptance Criteria

• Invalid Stellar addresses rejected with 400
• Valid addresses pass through unchanged
• Tests added and passing

Expected files to change

• backend/src/stellar/stellar.controller.ts
• backend/src/vault/vault.controller.ts
• backend/src/auth/auth.controller.ts

---

Commit: fix: return 400 on invalid Stellar address in backend routes
Branch: fix/backend-stellar-address-validation | Priority: MEDIUM | Difficulty: one-coffee

[Eniola3321]

@Eniola3321 has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

please can i work on the issue

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[jaymhorsh]

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Kindly assign I have ETC of 2-3 hrs, confident I'm the best candidate to resolve this issue as I have done on other issues previously on this repo

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[bytebinders]

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I would like to work on this issue.

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[bamiebot-maker]

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Kindly assign this issue to me

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