feat(blaze): unify sandbox API routes under /v1/sandboxes

[WeissonHan]

Problem

#2000 introduced /v1/sandboxes collection and item routes while retaining /v1/instances as compatibility aliases. The route families are still inconsistent: some operations are available under both names, while reset remains available only as POST /v1/instances/{id}/reset.

This forces clients to know operation-specific naming exceptions and leaves the canonical sandbox API incomplete.

Proposed behavior

Make /v1/sandboxes the canonical namespace for every sandbox operation.

• Add a /v1/sandboxes equivalent for every supported /v1/instances route.
• Route both names through the same handlers and preserve identical status codes, response bodies, idempotency, authorization, and side effects.
• Keep /v1/instances as a documented compatibility alias during an explicit migration window.
• Define the deprecation and eventual removal policy for /v1/instances before removing any route.

Acceptance criteria

• Inventory every current /v1/instances route and its /v1/sandboxes equivalent.
• Add missing sandbox-named routes, including POST /v1/sandboxes/{id}/reset.
• Preserve the reset contract established by fix(blaze)!: reject incomplete reset and pool use #2290: malformed IDs return 400, unknown IDs return 404, invalid lifecycle states return 422, and a running sandbox returns 501 without state or ownership changes until complete reset is implemented.
• Prove route equivalence through HTTP-level tests without duplicating business logic.
• Use /v1/sandboxes in primary examples and client-facing documentation.
• Mark /v1/instances as a compatibility namespace and document its migration window.
• Add release notes before removing any compatibility route.

Scope

This issue does not implement successful reset behavior and does not authorize immediate removal of /v1/instances. It tracks namespace consistency and a compatible migration plan.

Related: #2000, #2290, #2472.

[github-actions]

🔀 Issue Router

• Component: blaze
• Confidence: 99%
• Owners notified: @kongche-jbw
• No assignee was added; assignment records active implementation ownership.

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