Triage: Support path-style S3 endpoints

[time-attack]

Tracking the change proposed in #301 (by @lijunwang2024-ship-it).

We're moving this from a code PR to a tracked issue under CONTRIBUTING.md: feature ideas belong in adrs/ as a short human-written note and bugs as an issue, and we implement the underlying code from our side. Capturing the original proposal here so it isn't lost.

Original PR: #301
Reported by: @lijunwang2024-ship-it

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Summary

• add an opt-in S3_FORCE_PATH_STYLE runtime setting while preserving AWS defaults
• apply the setting to durable stores, AWS sandbox/deploy storage, and the Fly readiness probe
• verify both virtual-host and path-style request serialization

Testing

• node --test test/config.test.ts test/s3.test.ts test/blob-transfer.test.ts test/file-artifact-store.test.ts test/aws-sandbox.test.ts test/aws-deploy-provider.test.ts cli/test/fly-sandbox.test.ts
• npm run typecheck
• npm run format:check
• npm run lint -- --quiet
• npm run lint:ox
• git diff --check

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If we ship a fix or feature from this, credit @lijunwang2024-ship-it as co-author.