feat: migrate to A2UI protocol v1.0

[sethgw]

Summary

Tracking issue: a2ui-project/a2ui#1959. Also resolves a2ui-project/a2ui#1896 (explicit-null deletion in updateDataModel).

Migrates the repository's A2UI implementation from protocol v0.9/v0.9.1 to v1.0, following the v1.0 evolution guide and the specification/v1_0 schemas in google/A2UI.

Wire format (a2ui_core)

• All envelopes now use (and require) version: "v1.0"; the version constant is exported as a2uiProtocolVersion.
• createSurface: theme → surfaceProperties, plus optional inline components and dataModel for single-message UI instantiation.
• updateDataModel: explicit null deletion semantics — toJson() serializes "value": null instead of omitting it.
• Wire-level returnType removed from FunctionCall payloads (and from the expression parser output); boundaries/return types live in catalog definitions and are checked at runtime.

New v1.0 RPC messages

• Server → client: CallFunctionMessage (functionCallId, wantResponse, callFunction) and ActionResponseMessage (actionId, value XOR error).
• Client → server: A2uiFunctionResponse; A2uiClientAction gains wantResponse/actionId (generated when a component action sets wantResponse: true, with actionResponse values written back to the data model at responsePath); A2uiClientError supports functionCallId XOR surfaceId.
• The MessageProcessor executes remote calls with callableFrom boundary checks, rejecting clientOnly/unregistered functions with INVALID_FUNCTION_CALL, and exposes onFunctionResponse/onError channels (wired through SurfaceController to the outbound stream).

Catalogs

• functions serialized as an object map keyed by function name (v1.0 FunctionDefinition shape: FunctionCall validation schema + static returnType/callableFrom).
• $defs/surfaceProperties replaces theme (primaryColor removed); inline catalogs also emit $defs/anyComponent/$defs/anyFunction.
• Optional instructions field (genui embeds systemPromptFragments there, replacing external rules text).
• Catalog entity names validated against UAX Update GEMINI instructions for catalog items, add implementation descriptions. #31 identifier rules (isValidA2uiIdentifier).
• ClientFunction.callableFrom added (defaults to clientOnly).

Components & evaluation

• TextField.placeholder, Slider.steps, Video.posterUrl per the v1.0 basic catalog.
• @index built-in system function (with optional offset) scoped to template instantiation loops, in both the a2ui_core binder and the genui List/Row/Column template builders; the @ prefix is reserved and errors outside Collection Scope.
• basicCatalogId now points at https://a2ui.org/specification/v1_0/catalogs/basic/catalog.json.

Transport (genui_a2a)

• A2A extension URI bumped to https://a2ui.org/a2a-extension/a2ui/v1.0; capabilities/data-model envelopes namespaced under v1.0.
• Standardized application/a2ui+json MIME type constant.
• Incoming callFunction/actionResponse data parts routed to the message stream; outgoing action events carry actionId/wantResponse.

Docs, samples, tooling

• New migration guide: docs/usage/migration/migration_a2ui_v0.9_to_v1.0.md.
• CHANGELOG entries for a2ui_core, genui, and genui_a2a.
• All test fixtures, prompt-builder goldens, simple_chat integration samples, catalog_gallery samples (46), and composer constants updated to v1.0.

Not included (n/a to this codebase today)

• svgPath → path rename: genui's Icon uses a Material icon-name enum and has no custom-SVG definition object.
• Bridging genui ClientFunction execution into a2ui_core remote calls: all built-in genui functions are clientOnly, so remote calls to them are correctly rejected per spec; execution bridging can follow as a separate change.

Testing

• ./tool/run_all_tests_and_fixes.sh (repo-wide analyze + format + tests) passes.
• a2ui_core: 101 tests (was 69) — new coverage for callFunction/actionResponse round-trips, callableFrom rejection, explicit-null deletion, duplicate-surface errors, inline create, @index (offset, outside-scope error, nested inheritance), and UAX Update GEMINI instructions for catalog items, add implementation descriptions. #31 validation.
• genui: 279 tests, genui_a2a: 234 tests, catalog_gallery: 55 tests.

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