refactor(spec): Standardize on absolute URIs for schema and catalog references

### Description of the Inconsistency

There is an inconsistency in how schemas and catalogs are referenced across the A2UI repository.
Currently, the specification and examples mix **long/absolute URIs** (which tie identifiers to the repository's schema domains) and **short/relative URIs**.

#### 1. Long URIs used in Spec and Schema Files:
* In the official basic catalog [catalog.json](https://github.com/google/A2UI/blob/main/specification/v0_9/catalogs/basic/catalog.json), the `$id` and `catalogId` are set to `https://a2ui.org/specification/v0_9/catalogs/basic/catalog.json`.
* References to common types in [catalog.json](https://github.com/google/A2UI/blob/main/specification/v0_9/catalogs/basic/catalog.json) use the full URI: `https://a2ui.org/specification/v0_9/common_types.json#/$defs/ComponentCommon`.
* The protocol documentation [a2ui_protocol.md](https://github.com/google/A2UI/blob/main/specification/v0_9/docs/a2ui_protocol.md#L198) and examples use `https://a2ui.org/specification/v0_9/catalogs/basic/catalog.json` as the `catalogId`.

#### 2. Short/Relative URIs used in Code and Conformance Tests:
* Conformance test data such as [simplified_catalog_v09.json](https://github.com/google/A2UI/blob/main/agent_sdks/conformance/test_data/simplified_catalog_v09.json#L8) references common types using the short/relative filename: `"common_types.json#/$defs/ComponentCommon"`.
* The Web Core's `MessageProcessor` [message-processor.ts](https://github.com/google/A2UI/blob/main/renderers/web_core/src/v0_9/processing/message-processor.ts#L101) dynamically generates inline catalogs using the relative reference `"common_types.json#/$defs/ComponentCommon"`.

This inconsistency makes writing offline parser/validator registries fragile. A validator registry must map both long and short URIs to prevent validation failures, which goes against the design goal of catalog-agnostic validation.

### Proposed Solution

We should standardize on the **long/absolute version** of the catalog ID and schema URIs across all specifications, documents, and implementations.

1. **Standardize on Long URIs**:
* All `$ref` links inside catalogs, example messages, and generated schemas should use the full absolute URL representation (e.g. `https://a2ui.org/specification/v0_9/common_types.json#/$defs/...`) rather than relative filenames.
2. **Registry Mapping**:
* Platform-specific validator registries (such as in Java, Kotlin, or Web Core) should define standard base URL mappings so that these absolute URIs can be resolved locally offline without network access.