🚀 Release v2.3.0 — restore readable model picker (#122)

## Change Log / Overview

### 🚀 Features
- Restores the `LiteLLM: Show Available Models` picker from successful
per-group discovery snapshots.
- Displays models as `<group> :: <display name>` and copies the complete
`litellm-connector/<group>/<model>` selector.
- Keeps pricing and exact token limits in model details instead of the
primary picker identity.

### 🧪 Tests
- Adds regression coverage for discovery snapshots, readable picker
fields, and selector-prefixed copied IDs.

### 📚 Documentation
- Updates `CHANGELOG.md`, `README.md`, and `README.marketplace.md` for
v2.3.0.

### Related Issues, Builds, Pipeline Runs
- None specified.

### Pull Request Pre-check
- [x] Linting validation passed (`npm run lint`; existing warnings only)
- [x] Formatting validation passed (`npm run format`)
- [x] Unit testing passes (`npm run test:coverage`)
- [x] Documentation updated

Validation:
- `npm run compile` passed
- `npm run lint` passed with 31 existing warnings and 0 errors
- `npm run format` passed
- `npm run test:coverage` passed

Do not merge, tag, or publish from this branch. After squash-merging via
the web UI, tag `rel/v2.3.0` on `main` to trigger the release pipeline.