chore: set default provider type

[ronething]

Type of change:

• Bugfix
• New feature provided
• Improve performance
• Backport patches
• Documentation
• Refactor
• Chore
• CI/CD or Tests

What this PR does / why we need it:

set default provider type as 'apisix'

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[Copilot]

Pull Request Overview

This PR updates the default provider type from "apisix-standalone" to "apisix" to ensure consistency across configuration files and tests.

• Replace hard-coded strings with framework constants.
• Update manifests, constants, internal configuration, and sample configurations.
• Remove legacy ADC backend environment variable assignment.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/e2e/scaffold/apisix_deployer.go	Uses framework.ProviderType to determine the backend provider value.
test/e2e/scaffold/adc.go	Removes ADC_BACKEND env var assignment in the dumpResources function.
test/e2e/framework/manifests/ingress.yaml	Updates the default provider type in the ingress manifest.
test/e2e/framework/apisix_consts.go	Changes the default ProviderType constant to "apisix".
internal/controller/config/config.go	Replaces ProviderTypeStandalone with ProviderTypeAPISIX in config.
config/samples/config.yaml	Updates sample configuration to use "apisix" as the provider type.

Comments suppressed due to low confidence (1)

test/e2e/scaffold/adc.go:137

• The removal of the ADC_BACKEND environment variable assignment changes how the ADC backend is configured. Please confirm that this change aligns with the intended design and that ADC receives the correct provider information.

	}