fix: _parse_usage returns typed ImageUsage instead of RawUsage dict

[Kamilbenkirane]

Bug

_get_usage() in APIMixin (client.py:262) calls _parse_usage() expecting a RawUsage dict, then wraps it with self._usage_class(**raw).

Three modality-level overrides return typed ImageUsage objects instead of dicts:

• GeminiImagesClient._parse_usage() → ImageUsage(**usage, num_images=...)
• GoogleImagesClient._parse_usage() → delegates to Gemini strategy, returns ImageUsage
• OllamaImagesClient._parse_usage() → ImageUsage()

This causes a crash on Gemini image generation:

ImageUsage() argument after ** must be a mapping, not ImageUsage

Root cause

PR #151 ("centralize usage/finish_reason type conversion") introduced _get_usage() but these 3 overrides were never updated to return dicts.

The modality template also generates this same contract-violating pattern.

Fix

1. Align the 3 _parse_usage overrides to return dict[str, int | float | None]
2. Remove redundant _parse_usage and _parse_finish_reason from the modality template (_get_usage/_get_finish_reason already handle type conversion)