Fix DuplicateBlockError in multi-page apps with gr.render during reload

[gambletan]

Summary

• Fixes DuplicateBlockError when using gr.render in multi-page apps launched via gradio app.py (reload mode)
• After the initial Context.id = 0 reset in watchfn, ensures Context.id is advanced past all existing block IDs so that dynamically created blocks (from gr.render callbacks) don't collide with blocks from already-imported child page modules

Problem

When running a multi-page app with gradio app.py:

1. The original execution imports child page modules (e.g., page1.py, page2.py), creating blocks with sequential IDs starting from 0
2. watchfn resets Context.id = 0 and re-executes the main module
3. Since child modules are already in sys.modules, they are not re-imported — their blocks retain their original IDs
4. The re-executed root Blocks gets a new low ID, and Context.id ends up at a low value
5. When gr.render fires on first page load, new blocks get IDs that collide with existing block IDs, causing DuplicateBlockError

Fix

After the exec() call in watchfn, set Context.id to max(existing_block_ids) + 1 to prevent ID collisions.

Test plan

• Reproduce the issue from DuplicateBlockError in multi page app with render #12078 with the multi-page app example (gradio app.py) and verify the DuplicateBlockError no longer occurs on first page load
• Verify that page reload and hot-reload (code changes) still work correctly
• Verify that single-page apps with gr.render are unaffected

Closes #12078

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Install Gradio from this PR

pip install https://gradio-pypi-previews.s3.amazonaws.com/3f25d8c43047628203070bb31a642c154e581d9b/gradio-6.9.0-py3-none-any.whl

Install Gradio Python Client from this PR

pip install "gradio-client @ git+https://github.com/gradio-app/gradio@3f25d8c43047628203070bb31a642c154e581d9b#subdirectory=client/python"

Install Gradio JS Client from this PR

npm install https://gradio-npm-previews.s3.amazonaws.com/3f25d8c43047628203070bb31a642c154e581d9b/gradio-client-2.1.0.tgz

[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
gradio	patch

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• Fix DuplicateBlockError in multi-page apps with gr.render during reload

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✅ Changeset approved by @freddyaboulton

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[freddyaboulton]

Verified the fix thanks @gambletan