feat(google-vertex): add non-preview Gemini image models

[NielsAI]

Description

Add the Gemini image generation models under google-vertex, now that
they're GA (and 2.5 Flash Image as it's non-preview was also not yet present Vertex):

• gemini-2.5-flash-image (Nano Banana)
• gemini-3.1-flash-image (Nano Banana 2)
• gemini-3-pro-image (Nano Banana Pro)

Each inherits provider-agnostic facts via base_model from models/google/
and declares only the Vertex-specific cost and reasoning_options,
following the existing gemini-3.5-flash.toml pattern.

Pricing/limits verified against Vertex AI docs. Note output uses the
image-output token rate (repo convention for these models). Context for
3 Pro Image is 65,536/32,768 on Vertex (no 131K override).

[rekram1-node]

AUTOMATED REVIEW: I found a few data correctness issues that should be fixed before merge.

1. providers/google-vertex/models/gemini-3.1-flash-image.toml:1 inherits preview limits from google/gemini-3.1-flash-image-preview. The resolved Vertex entry becomes context = 65_536 and output = 65_536, but current Vertex docs list stable gemini-3.1-flash-image as context = 131_072 and output = 32_768. Please add a [limit] override.

2. providers/google-vertex/models/gemini-2.5-flash-image.toml:1-2 publishes reasoning = true via the base model plus reasoning_options = [], but Vertex/Gemini docs list gemini-2.5-flash-image as Thinking not supported. Please override reasoning = false and remove reasoning_options for this provider entry.

3. providers/google-vertex/models/gemini-3-pro-image.toml:1 leaves structured_output unset via the preview base, but current docs list stable gemini-3-pro-image structured outputs as supported. Please add structured_output = true.

4. providers/google-vertex/models/gemini-3-pro-image.toml:1 and providers/google-vertex/models/gemini-3.1-flash-image.toml:1 inherit preview release_date / last_updated values under stable GA IDs. Vertex docs list both stable IDs as GA with release date 2026-05-28; these should be overridden or represented by non-preview base metadata.

Checks run locally: bun validate passed, and git diff --check dev...HEAD passed.