fix(events): make undiverged honor module autoplanning

[krewenki]

what

• Make targeted undiverged checks use Atlantis project-impact resolution instead of only matching base branch changes against raw autoplan.when_modified patterns.
• Reuse the existing configured-project and autodiscovery logic so default branch divergence is evaluated the same way Atlantis decides whether a project is impacted.
• Add the WorkingDir.GetDivergedFiles() path needed to map base branch changes to impacted projects.
• Update the undiverged docs and add focused coverage around module and autodiscovery cases.

why

• The old targeted undiverged behavior could miss relevant base branch changes when a project was impacted through module autoplanning rather than a direct file match.
• That made the undiverged gate less accurate than Atlantis's own project selection logic, which is not a great split.
• This change keeps the gate aligned with the rest of Atlantis instead of reimplementing a simpler matcher in a separate path.
• If project-impact resolution cannot determine the answer, it falls back to a full divergence check so the behavior stays conservative.

tests

• I added unit coverage for configured-project module changes, autodiscovered project impacts, unrelated changes, and resolver-error fallback behavior.
• I ran GOCACHE=/tmp/go-build go test ./server/events/...
• I verified the server package still compiles with GOCACHE=/tmp/go-build go test ./server -run '^$'

references

• No linked issue.
• Docs updated in this PR: runatlantis.io/docs/command-requirements.md

[Copilot]

Pull request overview

Aligns targeted undiverged command requirement behavior with Atlantis’s project selection logic so that default-branch divergence is evaluated based on actual project impact (including module autoplanning and autodiscovery), rather than only raw when_modified glob matching.

Changes:

• Introduces an UndivergedProjectImpactResolver and wires it into DefaultCommandRequirementHandler for targeted undiverged checks.
• Extends WorkingDir with GetDivergedFiles() and refactors targeted divergence checks to reuse that logic.
• Adds focused unit tests for configured-project module impacts, autodiscovery impacts, unrelated changes, and resolver-error fallback; updates docs accordingly.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
server/server.go	Reuses a shared DefaultProjectFinder and wires the new impact resolver into the command requirement handler.
server/events/working_dir.go	Adds GetDivergedFiles() to WorkingDir and refactors targeted divergence checks to use shared diverged-file lookup.
server/events/command_requirement_handler.go	Adds resolver hook for targeted undiverged, with conservative fallback to full divergence check on errors.
server/events/undiverged_project_impact.go	Implements impact resolution mirroring Atlantis project selection (configured + autodiscovery + module autoplanning).
server/events/undiverged_project_impact_test.go	Adds unit coverage for module/autodiscovery impact scenarios and fallback behavior.
server/events/mocks/mock_working_dir.go	Updates generated mock to include GetDivergedFiles().
server/events/mock_workingdir_test.go	Updates generated in-package mock to include GetDivergedFiles().
runatlantis.io/docs/command-requirements.md	Documents updated targeted undiverged behavior and conservative fallback semantics.

[chenrui333]

Thanks @krewenki for your first contribution to Atlantis!