Align runtime validation behavior with v0.8 schema expectations for bindable fields

[selamw1]

Description

Addresses a runtime validation regression introduced in newer @a2ui/react versions when validating spec-compliant A2UI v0.8 payloads.

In @a2ui/react@0.8.0, bindable fields (such as MultipleChoice.selections) containing both path and a literal fallback (e.g., literalArray) were correctly validated, matching the official A2UI v0.8 catalog specification. Later releases introduced a strict refinement (exactlyOneKey) that failed at runtime with Must define exactly one property.

This PR replaces exactlyOneKey with atLeastOneKey for v0.8 bindable properties in @a2ui/web_core. This aligns runtime validation behavior with official v0.8 schema expectations while preserving backwards compatibility for producers emitting both data bindings and literal initial values.

Fixes: #1469

Pre-launch Checklist

• I signed the [CLA].
• I read the [Contributors Guide].
• I read the [Style Guide].
• I have added updates to the [CHANGELOG].
• I updated/added relevant documentation.
• My code changes (if any) have tests.
• If my branch is on fork, I have verified that scripts/e2e_test.sh passes.

[gemini-code-assist]

Code Review

This pull request updates the validation logic in the common types schema to allow at least one property instead of requiring exactly one. This change is applied to several schemas including StringValue, NumberValue, and BooleanValue. The review feedback identifies a logic mismatch in the DataValueMapItemSchema where the condition still checks for exactly one property despite the updated error message, and points out an outdated description string in the ActionSchema that needs to be updated for consistency.