BUILD-10724 migrate only when S3 is forced

[julien-carsique-sonarsource]

Summary

Change the default for import-github-cache from always true to context-aware:

• true when the S3 backend was explicitly forced via backend: s3 input or CACHE_BACKEND=s3 env var — the intentional GitHub→S3 migration scenario.
• false when the S3 backend was auto-detected (private/internal repository) — these repositories have always used S3 and have no GitHub cache entries to migrate.

Changes

• action.yml: cache-backend step now emits backend-source: forced|auto; import-mode step uses it as the smart default.
• README.md: updated inputs table and Migration Mode section to document the new default and resolution order.

Note on future default change

When S3 becomes the default backend for all repositories (including public), the auto-detection case will also need to check repository visibility (public → true, private → false). That change is deferred to when the default is actually switched.

Test coverage

gh-action_cache is public → without explicit backend it auto-detects GitHub, so all S3 scenarios require backend: s3 or CACHE_BACKEND=s3.
sonar-dummy is private → auto-detects S3, needed to test backend-source=auto behaviour.

#	Repo	S3 backend selection	Import mode	S3 cache	Expected	Covered
1	public	input	true (default)	miss	import from GitHub	✅ gh-action_cache
2	public	input	false (input)	miss	no import	✅ gh-action_cache
3	public	input	false (env)	miss	no import	✅ gh-action_cache
4	public	input	true (default)	hit	S3 content, no import	✅ gh-action_cache
5	private	env	true (default)	miss	import from GitHub	✅ sonar-dummy#573
6	private	auto	false (default)	miss	no import (new behaviour)	✅ sonar-dummy#573
7	private	auto	true (env)	miss	import from GitHub	✅ sonar-dummy#573
8	private	auto	true (input)	miss	import from GitHub	✅ sonar-dummy#573

[sonarqube-cloud-us]

SonarQube reviewer guide

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud]

SonarQube reviewer guide

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[bwalsh434]

Awesome! In theory, ths is must more efficient in number of operations executed 👌