feat: Add support for Seed node (CORE-295)

[yousef-rafat]

Add a simple seed node with a randomize button:

[coderabbitai]

📝 Walkthrough

Walkthrough

A new built-in extra node module, comfy_extras/nodes_seed.py, is added. It defines SeedNode, which registers with a single integer seed input and returns that value as its output. SeedExtension exposes the node through get_node_list, and comfy_entrypoint returns a SeedExtension instance. The module is added to init_builtin_extra_nodes() in nodes.py for startup autoloading.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 20.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: adding simple seed support.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The description matches the change set by describing the new simple seed node and its randomize-button UI.

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