Rebrand Vertex AI to Agent Platform

[takumiohym]

This PR renames "Vertex AI" to "Agent Platform" and reorganizes services under Models, Agents, and Notebooks in markdown cells and code comments, as requested. It also renames files and directories containing "vertex" and updates documentation URLs and file links where working links were found.

Changes Made

File and Directory Renaming

• Renamed files to remove "vertex" from their names. Updated references to the directory too.
• Renamed directories named pipeline_vertex to pipeline. Updated references to the directory too.

Content Updates

• Replaced "Vertex AI" and standalone "Vertex" with "Agent Platform" in markdown cells and code comments and docstrings in Python and shell scripts (not code itself) across notebooks and in README.md.
• Replaced "Vertex AI" and "Vertex" with "Agent Platform"
• Updated specific service names.
• Fixed links to renamed files in markdown cells across notebooks and .md files to restore functionality.

URL Updates

• Updated documentation URLs in files to use working links to use https://docs.cloud.google.com/gemini-enterprise-agent-platform/.
• Updated console URLs to use https://console.cloud.google.com/agent-platform.
• Kept other documentation URLs as vertex-ai if working links were not found.
• Left image URLs containing vertex-ai as they are.

Verification

• Verified that no code lines were changed (except for imports and paths in strings that needed updating due to renames).

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