Structured `error_type` taxonomy across AI service exceptions

[kilodesodiq-arch]

Problem Statement. app/ai-service/exceptions.py::AIServiceError carries a code
field. It's freeform — code is a string without validation.

Technical Context. Same as #41 on backend.

Expected Outcome. A ErrorCode enum in app/ai-service/schemas/errors.py. The
enum is the only allowed source of truth.

Acceptance Criteria.

• Tests pin all currently used codes; INVALID_CODE_USAGE raises at boot if any random
  string is passed.

Files or modules likely to be affected.
app/ai-service/exceptions.py,
app/ai-service/schemas/errors.py.

Dependencies. #41.

Difficulty. Easy
Estimated effort. S

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Backlog item #76 from `docs/maintainer-issue-backlog.md.

[OluRemiFour]

Hi Maintainer,
Interested in working on this issue. I'll implement the requested changes and ensure everything is properly tested before submission.

Payout wallet (Stellar): GALHFNUOYOY4S5CNRK7OI7NKS7UNWYGSJII2POQ6XQKWMUVLKZ2SAWMP GrantFox OSS campaign — smart escrow payout on merge.

[grantfox-oss]

🦊 GrantFox — @OluRemiFour has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #284)
2. Your PR will be reviewed by the ChainForgee maintainers

Good luck! Track your progress on GrantFox.

[OluRemiFour]

1. Closes Structured error_type taxonomy across AI service exceptions #284

Done

[OluRemiFour]

Problem Statement. app/ai-service/exceptions.py::AIServiceError carries a code field. It's freeform — code is a string without validation.

Technical Context. Same as #41 on backend.

Expected Outcome. A ErrorCode enum in app/ai-service/schemas/errors.py. The enum is the only allowed source of truth.

Acceptance Criteria.

• Tests pin all currently used codes; INVALID_CODE_USAGE raises at boot if any random
  string is passed.

Files or modules likely to be affected. app/ai-service/exceptions.py, app/ai-service/schemas/errors.py.

Dependencies. #41.

Difficulty. Easy Estimated effort. S

Backlog item #76 from `docs/maintainer-issue-backlog.md.

Done, Kindly merge