feat: expose Sharp encoder config options for image generation

[merlinnot]

Changes

• Adds codec-specific defaults to Astro’s built-in Sharp image service via image.service.config
• Supports jpeg, png, webp, and avif encoder option objects for compile-time image generation
• Applies these config values to Sharp’s format-specific encoders while keeping per-image quality as the higher-precedence override
• Preserves the existing animated GIF to WebP behavior, with loop: 0 remaining the default unless explicitly overridden

Testing

• Added unit tests for codec-specific encoder option resolution
• Verified config defaults are applied when no per-image quality is provided
• Verified per-image quality overrides config-level quality
• Verified jpg output maps to JPEG encoder defaults
• Verified animated GIF to WebP keeps the default loop: 0 unless overridden in config

Docs

Added public config docs for image.service.config.jpeg, image.service.config.png, image.service.config.webp, and image.service.config.avif

/cc @withastro/maintainers-docs for feedback!

[changeset-bot]

🦋 Changeset detected

Latest commit: 57b1808

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[codspeed-hq]

Merging this PR will improve performance by 10.37%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 17 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	Build: hybrid site (static + server)	8.8 s	8 s	+10.37%

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_{Comparing merlinnot:encoder (57b1808) with main (23d5244)}