* Add structured prompt upsampling to Ideogram4

Rewrite prompts into Ideogram4's native structured JSON caption before encoding,
opt-in via `prompt_upsampling=True` in `__call__` or the standalone
`upsample_prompt`. Upsampling is driven by a generative `text_encoder`
(`Qwen3VLForConditionalGeneration`, which carries the LM head); the head-less
`Qwen3VLModel` is still supported for plain conditioning, and `upsample_prompt`
raises an instructive error when the encoder cannot generate. Captions are
schema-constrained via `outlines` when installed, and the modular pipeline gains
a matching prompt-enhancer block.

* Remove LM-head grafting; modular block uses a generative text_encoder

Drop `graft_lm_head` and drive the modular `Ideogram4PromptUpsampleStep` off a
generative `text_encoder` (`Qwen3VLForConditionalGeneration`), matching the
standard pipeline: guard with `can_generate()` and an instructive error, and
build the outlines logits processor lazily. Updates the copied
`_get_text_encoder_hidden_states` to resolve the decoder for both encoder
classes.

* Style docstrings with doc-builder; mark prompt strings docstyle-ignore

Reflow the Ideogram4 prompt-enhancer docstrings to the 119-col doc-builder
style, and add `# docstyle-ignore` to the functional `CAPTION_SYSTEM_MESSAGE`
and `CAPTION_USER_TEMPLATE` strings so the styler doesn't rewrap them (matching
Flux2's `system_messages.py`).

* Use an Ideogram4PromptEnhancerHead component for prompt upsampling

Add `Ideogram4PromptEnhancerHead`, a small `ModelMixin` holding the Qwen3-VL LM
head, as an optional `prompt_enhancer_head` pipeline component. Upsampling loads
the head via a normal `from_pretrained` (its own repo, or bundled later) instead
of an in-pipeline download, and grafts it onto the shared head-less `text_encoder`
body so no second 8B body is loaded. Both the standard and modular pipelines build
the generative model from `text_encoder` + the head; `upsample_prompt` raises an
instructive error when the head component is absent.

* Update src/diffusers/pipelines/ideogram4/pipeline_ideogram4.py

Co-authored-by: dg845 <58458699+dg845@users.noreply.github.com>

* Update src/diffusers/pipelines/ideogram4/pipeline_ideogram4.py

Co-authored-by: dg845 <58458699+dg845@users.noreply.github.com>

* Apply suggestion from @apolinario

* Apply suggestion from @dg845

Co-authored-by: dg845 <58458699+dg845@users.noreply.github.com>

* Apply suggestion from @apolinario

* Fix trailing whitespace

* docs: add prompt-upsampling examples (remote API + local head) for Ideogram4

---------

Co-authored-by: dg845 <58458699+dg845@users.noreply.github.com>