[flutter_svg] add imageBuilder property to SvgPicture

[suojae]

related with flutter/flutter#182635

This PR exposes the imageBuilder support added to VectorGraphic in #11094 through SvgPicture.

SvgPicture already supports placeholderBuilder and errorBuilder, but it did not provide a success-state builder. Without this API, callers have to wrap SvgPicture externally, which also wraps the placeholder and error states.

Adding imageBuilder lets callers decorate or wrap only the successfully loaded SVG, while keeping loading and error UI separate.

Alternatives considered

• I chose imageBuilder because it follows the success-state wrapping pattern used by widgets like CachedNetworkImage

Changes

• Added SvgImageWidgetBuilder.
• Added imageBuilder to all SvgPicture constructors.
• Forwarded imageBuilder to createCompatVectorGraphic.
• Added tests for successful load and placeholder states.
• Bumped flutter_svg to 2.3.0 and vector_graphics to ^1.2.0.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets.
• I linked to at least one issue in the description above.
• I followed the version and CHANGELOG instructions.
• I updated/added any relevant documentation.
• I added new tests to check the change I am making.
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request adds an imageBuilder property to the SvgPicture widget, enabling custom wrapping of successfully loaded SVGs. The implementation includes a new SvgImageWidgetBuilder typedef, updates to all SvgPicture constructors, and a version bump to 2.3.0. The vector_graphics dependency is also updated to 1.2.0, supported by new unit tests for the imageBuilder functionality. I have no feedback to provide.

[gaaclarke]

Can you spend some more time explaining in the description why you think we need this API. What problem is it solving and what alternatives did you consider?

[gaaclarke]

(requested more information in description above)

[suojae]

Oh, sorry for the thin description. I've updated the PR description to explain why this API is needed, what problem it solves, and the alternatives I considered.

[gaaclarke]

Okay, thanks for expanding on why we are doing this. While there was technically a way to achieve this before the PR, it required a lot of boilerplate code (like ByteLoader subclass). It's unreasonable to expect all users to implement that. This is consistent with existing APIs and sounds good to me.

My only concern is I believe the new tests operate with the svg cache so there can be bleeding state between tests. I think it would be prudent to clear the cache between tests:

  setUp(() {
    svg.cache.clear();
  });

[gaaclarke]

Also a friendly reminder: when you want me to re-review things, please press the "re-request review" button otherwise I may miss it.