provider: add Modelsell

[modelsell]

Description:

• Adds Modelsell to the provider registry and provider metadata with the fixed OpenAI-compatible API base https://modelsell.com/v1.
• Supports bearer-token Chat Completions, including streaming requests, through Portkey’s existing OpenAI-compatible transforms.
• Preserves Modelsell’s dynamic catalog: no model IDs are added to models.json, and the existing provider proxy maps GET /v1/models to https://modelsell.com/v1/models.
• Adds focused tests for registration, authentication, chat routing, and dynamic model discovery.

Fixes #1750

Disclosure: I represent Modelsell.

No live authenticated Modelsell API call was performed.

Tests Run/Test cases added:

• npx jest src/providers/modelsell/api.test.ts --runInBand (3 tests passed)
• npm run build
• npm run format:check
• Pre-push build and local gateway startup smoke check
• git diff --check

The repository-wide Jest command also runs existing test suites that currently fail on missing .creds.json, stale relative mocks, and pre-existing TypeScript errors; the focused Modelsell suite passes independently.

Type of Change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)