fix: stop scoping LATEST dependency resolution to --branch flag @W-21617695@

[WillieRuemmele]

Summary

• When --branch is passed to sf package version create, LATEST dependency resolution now falls back to the null branch (main/released versions) if the dependency has no versions on the specified branch
• Previously, this would fail with NoReleaseVersionFoundForBranchError — now it gracefully resolves from the unscoped branch
• Dependencies that DO have versions on the branch still resolve from that branch (existing behavior preserved)
• Dependencies with an explicit branch in their config still use that branch (existing behavior preserved)

Fixes forcedotcom/cli#3517

Work Item

@W-21617695@: LATEST dependency resolution incorrectly scoped to --branch for all packages

Proof of Work

• Tests: 275 passing, 0 failing
• Lint: clean (0 errors)
• Type check: clean

Test plan

• Run sf package version create --branch <branch> with dependencies using LATEST that don't have versions on the specified branch — should now succeed by falling back to null branch resolution
• Run sf package version create --branch <branch> with dependencies that DO have versions on the branch — should resolve LATEST from that branch (existing behavior)
• Run sf package version create --branch <branch> with dependencies that have an explicit branch in their config — should still resolve against that explicit branch
• Run sf package version create (no --branch flag) with LATEST dependencies — behavior unchanged (resolves against null branch)
• Run with dependency.branch set to empty string '' — should resolve against null branch (existing behavior preserved)