clarify docs on a2ui metadata being on the message

[wrenj]

Description

This pull request clarifies the technical documentation regarding the placement of a2uiClientCapabilities within A2A messages. By standardizing the language across SDK guides and protocol specifications, it ensures that developers correctly implement the required metadata structure, reducing ambiguity in the communication protocol.

Highlights

• Documentation Clarification: Updated multiple documentation files to explicitly state that a2uiClientCapabilities must be included in the metadata field of the A2A Message.
• Consistency Improvements: Standardized the terminology across various SDK and protocol specification documents to ensure developers have a clear understanding of where to place capability metadata.

[gemini-code-assist]

Code Review

This pull request updates several documentation files to clarify that the a2uiClientCapabilities object is located specifically within the metadata field of an A2A Message. The feedback identifies a redundant and grammatically incorrect phrase in custom_catalog_changes.md ("in the metadata field of the A2A Message's metadata field") and suggests simplifying it.

[gspencergoog]

@wrenj are you still working on this PR?

[wrenj]

hey @gspencergoog , I'm OOO and traveling for the next 3 weeks, feel free to take over this PR or I'll try to submit it in a few weeks when I'm not traveling, sorry for not getting this in before I left. It was needed for GE since they were putting a2uiClientCapabilities in the sendMessageRequest.metadata instead of message.metadata so I wanted ot make taht clear.

[gspencergoog]

No worries, I was just looking at the PR backlog and wanted to know status. I'll probably try and resolve conflicts and commit this.