cleanup(bootstrapper): expose DEFAULT_BG_THREADS as public class constant

[rd4398]

Make DEFAULT_BG_THREADS a public class constant on Bootstrapper
instead of a private module-level _DEFAULT_BG_THREADS in _types.py.
The CLI layer now references Bootstrapper.DEFAULT_BG_THREADS for the
--bg-threads default, keeping the thread-count knowledge with the
bootstrapper while giving it a clean public API surface.

Closes: #1246

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Review details

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 1776cbbd-5ab9-481f-bdb4-bf39027fd7f6

📥 Commits

Reviewing files that changed from the base of the PR and between 182e0e1 and 39d29be.

📒 Files selected for processing (4)

• src/fromager/bootstrapper/__init__.py
• src/fromager/bootstrapper/_bootstrapper.py
• src/fromager/bootstrapper/_types.py
• src/fromager/commands/bootstrap.py

📝 Walkthrough

Walkthrough

The bootstrapper package now exports only Bootstrapper. Bootstrapper computes its default background thread count from the CPU count and accepts an optional override. Both bootstrap CLI commands use Bootstrapper.DEFAULT_BG_THREADS for their --bg-threads defaults.

Estimated code review effort: 1 (Trivial) | ~3 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The PR addresses [#1246] by removing _DEFAULT_BG_THREADS from the package export surface and keeping the CLI default working.
Out of Scope Changes check	✅ Passed	The changes are confined to the bootstrapper default-thread cleanup and the CLI consumer update.
Title check	✅ Passed	The title clearly describes the main change: moving DEFAULT_BG_THREADS to a public Bootstrapper class constant.
Description check	✅ Passed	The description matches the changeset and explains the API and CLI default updates.

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/fromager/bootstrapper/_bootstrapper.py`:
- Line 81: Update the _num_bg_threads assignment in the bootstrapper initializer
to distinguish an explicit num_bg_threads value of 0 from None or other absent
values, so 0 continues to normalize to one worker while omitted values use
DEFAULT_BG_THREADS. Preserve the existing max(1, ...) lower-bound behavior.

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• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

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Run ID: 672a3a07-8548-4585-ac7e-f3d445ff22af

📥 Commits

Reviewing files that changed from the base of the PR and between 5c92caa and 182e0e1.

📒 Files selected for processing (4)

• src/fromager/bootstrapper/__init__.py
• src/fromager/bootstrapper/_bootstrapper.py
• src/fromager/bootstrapper/_types.py
• src/fromager/commands/bootstrap.py

🚧 Files skipped from review as they are similar to previous changes (1)

• src/fromager/bootstrapper/init.py

[LalatenduMohanty]

The PR looks good to me. Once you fix the PR description we should be able to merge it.

[smoparth]

Making DEFAULT_BG_THREADS as public is good choice. I just mentioned small nitpick above.