feat(sdk): Creation of the SDK to gave client-python better scope/limits

[Kakudou]

Decision

We are restructuring the SDK following the DDD + Light Hexagonal architecture validated in the PoC branch.
This is not a proposal the architecture is proven and the design decisions are settled. (#199 , #301)

This issue tracks building the production SDK properly on a new branch, informed by the PoC.

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The Split

The monolith client-python is divided into two packages with distinct responsibilities:

│ Package │ Role │
| --- | --- |
│ xtm-oaev-sdk │ Shared core: domain logic, data contracts, protocols, configuration, signature pipeline │
│ pyoaev │ Transport layer: HTTP client, connector runtime, OpenAEV concrete implementation │

xtm-oaev-sdk  has no dependency on  pyoaev .  
pyoaev depends on  xtm-oaev-sdk  and implements its protocols.

The actual work not gonna be to 'rewrite the world in a different shape', simply split it in two.
So that, later one, we gonna have more freedom/ease to refactor/enhanced the code.

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Architecture

Three explicit layers inside  xtm_oaev_sdk :

public/       ← flat re-export surface;
contracts/    ← stable ports: protocols, data models, enums
_core/        ← private implementations

Dependency direction is strict and one-way.

contracts/  defines the interfaces.  
_core/  implements them.  
contracts/  never imports from  _core/ .

This is the Light Hexagonal pattern:  
contracts/  is the ports layer;  
_core/  is the adapters layer.
Internal layout of  _core/  may change freely;

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Shim Strategy

During migration,  pyoaev  keeps backward-compatible  __getattr__ -based shims that proxy old import paths to xtm-oaev-sdk  and emit  DeprecationWarning

# Old path, still works, warns
from pyoaev.helpers import ssl_cert_chain
from pyoaev.exceptions import OpenAEVError

# New path, clean
from xtm_oaev_sdk import ssl_cert_chain
from xtm_oaev_sdk import OpenAEVError

Once all connectors are migrated, the shim module is removed from  pyoaev .  
xtm-oaev-sdk  becomes the sole source.

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Status and Next Steps

Next:

• Create new branch; build  xtm-oaev-sdk  as production code from scratch, informed by the PoC
• Update  pyoaev  to declare  xtm-oaev-sdk  as a dependency and remove duplicated symbols
• Delete PoC branch once new branch is created and populated
• Migrate connectors to import from  xtm_oaev_sdk  directly.
• Remove shim module from  pyoaev  once connector migration is complete.