fix(router): allow from_file header to be propagated

[Noroth]

File headers were missing in the validation for the factory resolver. This PR adds this and also adds missing tests

Summary by CodeRabbit

• New Features

  • Request headers can now be set using values sourced directly from external files, providing greater flexibility in configuring headers across your routing setup.

• Tests

  • Added integration and unit tests to verify file-based header sourcing functionality works correctly.

• Bug Fixes

  • Enhanced validation for header set rules to ensure all required configuration fields are present and properly configured before processing.

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.
• If applicable, I have provided instructions for reviewers for manually validating my changes.
• Documentation has been updated on https://github.com/wundergraph/docs-website.
• I have read the Contributors Guide.

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This project follows the principles of the Open Source AI Manifesto. Please ensure your contribution aligns with its principles.

[coderabbitai]

Walkthrough

Adds FromFile to the HeaderRuleOperationSet invalid-rule guard in PropagatedHeaders, a unit test for Set+FromFile in TestPropagatedHeaders, and an integration test TestHeaderSetFromFile that writes a header value to a temp file and verifies the router returns that value for both global and subgraph-scoped request header rules.

Changes

FromFile header set validation and test coverage

Layer / File(s)	Summary
PropagatedHeaders FromFile validation fix and unit test router/core/header_rule_engine.go, router/core/header_rule_engine_test.go	Extends the HeaderRuleOperationSet invalid-rule condition to also reject rules where FromFile is absent alongside Value, ValueFrom, and Expression. Adds a TestPropagatedHeaders subtest confirming Set+FromFile rules yield the header name and no compiled regexps.
Integration test for file-sourced header set rules router-tests/protocol/header_set_test.go	Adds os/path/filepath imports and TestHeaderSetFromFile, which writes a header value to a temp file, configures RequestHeaderRule.FromFile, and asserts the router returns the file-derived value for both All.Request (global) and Subgraphs[...].Request (subgraph-specific) rules.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• wundergraph/cosmo#2908: Implements the FromFile header source feature in header_rule_engine.go that this PR's validation fix and tests directly build upon.

🚥 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 pull request title accurately summarizes the main change: adding support for propagating file-based headers through validation in the router's header rule engine.
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	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

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[github-actions]

Router image scan passed

✅ No security vulnerabilities found in image:

ghcr.io/wundergraph/cosmo/router:sha-b122c9906ea92c1dab641259cc36d5e018272225

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.42%. Comparing base (7532f9a) to head (57560cc).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2979      +/-   ##
==========================================
+ Coverage   65.34%   66.42%   +1.08%     
==========================================
  Files         335      258      -77     
  Lines       48194    27557   -20637     
  Branches     5371        0    -5371     
==========================================
- Hits        31491    18306   -13185     
+ Misses      16677     7792    -8885     
- Partials       26     1459    +1433     

Files with missing lines	Coverage Δ
router/core/header_rule_engine.go	88.21% <100.00%> (ø)

... and 592 files with indirect coverage changes

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