[CT-16] Write integration tests for POST /submit

[mftee]

Problem

The POST /submit endpoint (CT-01) has no test coverage. Once implemented it can regress silently — a broken submission handler would prevent all new documents from being anchored on Stellar.

Proposed Solution

Write integration tests using axum-test and httpmock inside contract/module/tests/submit_tests.rs.

Acceptance Criteria

• Test: valid 64-character hex hash returns 200 with a SubmitResponse containing success=true and a non-null transaction_id
• Test: hash shorter than 64 characters returns 400 with a descriptive error message
• Test: hash containing non-hex characters returns 400
• Test: empty document_hash field returns 400
• Test: submitting the same hash twice returns 409 Conflict with an already submitted error
• Test: Stellar client failure returns 500 (mock the StellarClient to return an error)
• All test files live inside contract/module/tests/

[EneGab]

@EneGab has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer, I would like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @EneGab to this issue.

[lynaDev2]

@lynaDev2 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

can i work on this

ℹ️ Repo Maintainers: To accept this application, review their application or assign @lynaDev2 to this issue.

[grantfox-oss]

🦊 GrantFox — @lynaDev2 has been assigned to this issue!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #548)
2. Your PR will be reviewed by the CodeGirlsInc maintainers

Good luck! Track your progress on GrantFox.

[drips-wave]

Congratulations, @lynaDev2! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @lynaDev2: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox!

@lynaDev2's PR #878 was approved and merged by @mftee.

🏆 @lynaDev2: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (331 total points). Track your full progress on GrantFox.

👏 Great work, @lynaDev2! Keep contributing to CodeGirlsInc.

[drips-wave]

This issue has been completed by @lynaDev2 as part of the Stellar Wave Program's 6th Wave 🥳

😎 @lynaDev2: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

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