Fully deprecate oembed endpoints

[juliawu]

Description

This PR deprecates and completely removes all implementations, routes, blueprints, templates, and experimental guides for oembed from the codebase.

The oembed and /chart endpoints were originally designed to provide content management services with a mechanism to rich-embed Data Commons web components, but are no longer supported. We got reports of XSS vulnerabilities found by an external reporter, and the decision was made to remove these endpoints instead of trying to fix it.

Changes Made

1. Core Server Blueprint Changes

• Modified server/__init__.py: Removed oembed and oembed_chart blueprint imports and application registrations from register_routes_common().

2. Backend File & Directory Deletions

• Deleted server/routes/oembed/chart.py (implemented the /chart embed-friendly page).
• Deleted server/routes/oembed/oembed.py (implemented the /oembed API handler returning JSON/XML properties).
• Deleted server/routes/oembed/ directory entirely.

3. Frontend Templates & Examples Deletions

• Deleted server/templates/oembed_chart.html (the dedicated Jinja template providing alternate application/json+oembed and text/xml+oembed discovery headers).
• Deleted experimental/oembed/oembed_example.html and its parent directory (a developer/user playground guide documenting oembed usage).

Verification

1. Lint and Formatting

• Ran ./run_test.sh -f to perform fully automated code style adjustment, verifying Python files conform to yapf and isort standards.

2. Routing Validation

• Run ./run_server.sh to spin up a local server
• Confirmed that zero oembed or /chart routes are registered on the server. Any attempts to hit these endpoints now trigger standard 404 Not Found responses.

[gemini-code-assist]

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[juliawu]

/gemini review

[gemini-code-assist]

Code Review

This pull request removes the oEmbed functionality from the server. Specifically, it deletes the oEmbed and chart route definitions, removes their blueprint registrations in the main application entry point, and deletes the associated HTML templates and documentation. I have no feedback to provide.

[nick-nlb]

LGTM - thanks for this Julia!