GUI advertises TokenBijectionConverter without its required tokenizer

@akshay-babbar is already working on this.

Since Sep 8, 2026.

#### Describe the bug

The frontend converter picker advertises `TokenBijectionConverter`, but the GUI/API construction path cannot provide its required keyword-only `tokenizer`. The UI exposes only the optional `seed` parameter, so every preview attempt returns HTTP 500 and the converter cannot complete its primary GUI workflow.

`TokenBijectionConverter.__init__` requires a tokenizer with `get_vocab()`, while the converter catalog exposes only string-coercible parameters. The required tokenizer is therefore omitted from the catalog response before the frontend renders the form:

- [`converter_service.py` catalog projection](https://github.com/microsoft/PyRIT/blob/e9277c45ab136b39f9aca12745bb45746f7d9963/pyrit/backend/services/converter_service.py#L85-L108)
- [`converter_service.py` instance creation](https://github.com/microsoft/PyRIT/blob/e9277c45ab136b39f9aca12745bb45746f7d9963/pyrit/backend/services/converter_service.py#L131-L156)
- [`TokenBijectionConverter` constructor](https://github.com/microsoft/PyRIT/blob/e9277c45ab136b39f9aca12745bb45746f7d9963/pyrit/converter/bijection_converter.py#L337-L369)

**Recommended direction:** exclude or clearly disable this converter in the GUI for now rather than silently selecting a fixed tokenizer. The converter is documented as using the target model's tokenizer vocabulary, and the GUI supports heterogeneous targets without a serializable tokenizer configuration. A hard-coded tokenizer could therefore be incorrect or misleading for the active target and may also introduce an implicit model download.

A durable GUI implementation should make tokenizer choice explicit and backend-managed, for example through a supported tokenizer identifier/catalog with validation and a clear relationship to the active target. Until that exists, the catalog/UI should generically identify converters with required non-GUI-constructible parameters and avoid offering an action that can only fail.

#### Steps/Code to Reproduce

1. Activate any writable text target.
2. Open Chat and enter a prompt.
3. Open the converter panel and select `TokenBijectionConverter`.
4. Expand Parameters; observe that only `seed` is available.
5. Choose Preview.

Equivalent API behavior:

```text
POST /api/converters
{"type":"TokenBijectionConverter","params":{}}

HTTP 500
Failed to create converter: TokenBijectionConverter.__init__() missing 1 required keyword-only argument: 'tokenizer'
```

#### Expected Results

The GUI should never present an impossible converter workflow. Until supported tokenizer selection exists, `TokenBijectionConverter` should be excluded or shown as unavailable with an actionable explanation, and Preview should not issue a request that can only fail.

If GUI tokenizer support is added, the user should select a supported backend-managed tokenizer identifier, incompatible or unavailable choices should fail validation before creation, and a valid selection should preview successfully without an implicit arbitrary default.

#### Actual Results

The converter appears selectable and looks ready to use, but Preview makes two failed create attempts and reports:

```text
Failed to create converter: TokenBijectionConverter.__init__() missing 1 required keyword-only argument: 'tokenizer'
```

Digit and Letter Bijection converters preview and apply successfully in the same workflow, isolating the failure to Token Bijection construction.

#### Screenshots

N/A. The exact visible error and API response are included above.

#### Versions

- OS: Windows_NT
- Browser: Playwright Chromium (`@playwright/test` 1.62.1)
- Python: 3.14.4
- PyRIT: `1.1.0.dev0`, `main` at `e9277c45ab136b39f9aca12745bb45746f7d9963`
- Python package versions: repository `uv.lock` at the tested commit