[codex] Add Modelsell provider preset

[modelsell]

Description

Adds Modelsell as a featured built-in OpenAI-compatible provider preset.

The integration includes:

• the Modelsell provider enum and registry definition
• https://modelsell.com/v1 as the default API host
• dynamic model discovery through https://modelsell.com/v1/models
• API key, documentation, and website links
• provider icon and display-name wiring
• a provider factory regression test

Models remain dynamically fetched and are not hardcoded.

Closes #3811

Additional Notes

Validation performed:

• Build passed.
• Typecheck passed after the build generated the ignored route tree.
• Focused provider/model tests passed: 13 tests.
• Changed-file lint reported only 3 existing warnings in src/shared/providers/index.ts.
• Full lint still has unrelated baseline failures: 11 errors and 984 warnings.

No live authenticated Modelsell API call was performed.

Screenshots

Not applicable; this change adds a provider preset using the existing settings UI.

Contributor Agreement

By submitting this Pull Request, I confirm that I have read and agree to the following terms:

• I agree to contribute all code submitted in this PR to the open-source community edition licensed under GPLv3 and the proprietary official edition without compensation.
• I grant the official edition development team the rights to freely use, modify, and distribute this code, including for commercial purposes.
• I confirm that this code is my original work, or I have obtained the appropriate authorization from the copyright holder to submit this code under these terms.
• I understand that the submitted code will be publicly released under the GPLv3 license, and may also be used in the proprietary official edition.

[x] I have read and agree with the above statement.

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