feat(cli): support multiple constraints files via repeated -c flag

[smoparth]

• Add multiple=True to the -c/--constraints-file global CLI option
• Update WorkContext to load multiple constraint files (local paths or remote URIs)
• Rewrite pip_constraint_args to write a single merged constraints file for uv

Closes: #1096

Pull Request Description

What

Why

• PR follows CONTRIBUTING.md guidelines

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: de787ccc-abe3-439b-80c7-f0e56d480528

📥 Commits

Reviewing files that changed from the base of the PR and between dd7009b and ab7325e.

📒 Files selected for processing (4)

• src/fromager/__main__.py
• src/fromager/context.py
• tests/test_cli.py
• tests/test_context.py

📝 Walkthrough

Walkthrough

This PR implements support for multiple constraints files in the fromager CLI. The --constraints-file option now accepts multiple values via Click's multiple=True parameter. WorkContext is updated to store a list of constraint file URIs instead of a single URI, loading each into self.constraints. The pip_constraint_args property now generates a merged input-constraints.txt file from all loaded constraints and returns the appropriate --constraint flag. HTTP/remote URL fetching logic for constraints is removed. CLI and context tests are added to validate the merging behavior with both single and multiple constraint files.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: adding support for multiple constraints files via the repeated -c flag in the CLI.
Description check	✅ Passed	The description is related to the changeset, mentioning multiple constraints files, WorkContext updates, and merging constraints, though it lacks detailed explanation of the implementation.
Linked Issues check	✅ Passed	The PR implements both objectives from issue #1096: updating the CLI to accept multiple constraints files (multiple=True on -c option) and updating WorkContext to load and merge constraints from all provided files.
Out of Scope Changes check	✅ Passed	All changes are in scope: CLI option update, WorkContext refactoring to handle multiple files, pip_constraint_args adaptation, and test coverage for the new functionality.

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[coderabbitai]

Actionable comments posted: 2

🤖 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/context.py`:
- Around line 143-150: The current logic skips writing input-constraints.txt if
it already exists, which can leave stale constraints in self.work_dir; change
the block that computes path_to_constraints_file and the lines list to always
write the merged constraints file (i.e., remove the exists() guard) so that
path_to_constraints_file.write_text("".join(lines)) runs on every invocation;
use the existing self.constraints.get_constraint(constraint_name) loop to
rebuild the contents before writing to input-constraints.txt.

In `@tests/test_context.py`:
- Around line 59-71: Update the test_pip_constraints_args_multiple_files
assertion to verify both constraint files contributed by checking the merged
file contains the numpy lower and upper bounds together and that numpy appears
only once: after ctx.setup() read merged_path and assert "numpy>=1.24" in
content, assert "numpy<2.0" in content, assert content.count("numpy") == 1, and
still assert "requests" is present and ctx.pip_constraint_args equals
["--constraint", os.fspath(merged_path)].

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Fix all unresolved CodeRabbit comments on this PR:

• 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

Plan: Pro

Run ID: 72076441-bf4d-447a-a6fa-ec6931bdafd3

📥 Commits

Reviewing files that changed from the base of the PR and between b30aae7 and dd7009b.

📒 Files selected for processing (4)

• src/fromager/__main__.py
• src/fromager/context.py
• tests/test_cli.py
• tests/test_context.py