Feat/incremental catalog loading

[leoafarias]

Issue: #946

Description

Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.

List which issues are fixed by this PR. For larger changes, raising an issue first helps reduce redundant work.

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I have added sample code updates to the changelog.
• I updated/added relevant documentation (doc comments with ///).
• If my PR is a fork PR, I've checked that [e2e tests] passed.

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[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:a2ui_core	0.0.1-dev002	already published at pub.dev
package:genai_primitives	0.2.4-dev001	ready to publish	genai_primitives-v0.2.4-dev001
package:genui	0.9.1	ready to publish	genui-v0.9.1
package:genui_a2a	0.9.0	already published at pub.dev
package:json_schema_builder	0.1.4	ready to publish	json_schema_builder-v0.1.4

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[gemini-code-assist]

Code Review

This pull request introduces an incremental catalog prompt mode to optimize system prompts for LLMs. It adds CatalogContext to manage a compact catalog manifest and load detailed component schemas and examples on demand via a new loadCatalogItems tool. It also updates PromptBuilder to support both full-schema and incremental modes, adjusting the system prompt structure and tool policies accordingly. The reviewer's feedback suggests improving robustness by supporting case-insensitive catalog item lookups, enhancing error handling during example JSON decoding to provide clearer debugging context, and adding corresponding unit tests.