fix(cmd): fix file descriptor leak from defer in loop

[SVilgelm]

Description

Extract file creation and writing into writeFile() helper so that defer file.Close() runs per-call instead of accumulating until Run() returns. With many mock files, the previous code could exhaust OS file descriptor limits.

• Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Version of Go used when building/testing:

• 1.25

How Has This Been Tested?

1. go test ./... - no issues
2. e2e/run_all.sh (fails on v3 as well)

RUNNING /Users/svunion/projects/mockery/e2e/test_missing_interface/run.sh
task: [mocks.generate.custom] go run .
2026-04-02T09:02:44.895693000-07:00 INF Starting mockery config-file=/Users/svunion/projects/mockery/e2e/test_missing_interface/.mockery.yml version=v0.0.0-dev
2026-04-02T09:02:44.895769000-07:00 INF Parsing configured packages... version=v0.0.0-dev
2026-04-02T09:02:45.008703000-07:00 INF Done parsing configured packages. version=v0.0.0-dev
2026-04-02T09:02:45.008739000-07:00 ERR interface not found in source. View the linked documentation for possible reasons why this might occur. docs-url=https://vektra.github.io/mockery/v0.0/faq/#error-interface-not-found-in-source interface=InterfaceDoesntExist package-path=github.com/vektra/mockery/v3/internal version=v0.0.0-dev
exit status 1
task: Failed to run task "mocks.generate.custom": exit status 1

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[Copilot]

Pull request overview

This PR addresses a potential OS file descriptor exhaustion in RootApp.Run() by ensuring file handles are closed per generated mock file rather than being deferred until the end of Run().

Changes:

• Introduces a writeFile() helper to encapsulate create/write/close behavior.
• Replaces the per-iteration defer file.Close() in the mock-generation loop with a call to writeFile().

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