Fix attach_session URL to match sprites-go and sprites-py

[jhgaylor]

Summary

Sprites.attach_session/3 was building URLs as /v1/sprites/:name/exec?session_id=X, but the server expects the path-based form /v1/sprites/:name/exec/:session_id — the same shape both sprites-go and sprites-py use.

Symptom

attach_session would return {:ok, command} because the WebSocket upgrade succeeded — the server saw the bad URL as a fresh exec with empty command. The originally detached process kept running on the sprite, but no output ever flowed to the new connection. End result: it was impossible to recover an in-flight detachable: true command after a client-side crash.

Fix

When :session_id is set in opts:

• Build the attach URL with the session id in the path, matching the other SDKs.
• Skip spawn-only query params (path/cmd/env/dir/detachable) — the server ignores them on attach since they belong to the original spawn.
• stdin/tty are still forwarded since they're meaningful for the attached side.

Test plan

• Existing test suite still passes (24 tests, 1 unrelated boilerplate failure).
• Verified end-to-end with a probe:
  1. Spawn bash -c "echo BEFORE; sleep 20; echo AFTER" with detachable: true.
  2. Hard-kill the Command GenServer mid-sleep.
  3. list_sessions shows the detached session with is_active: false.
  4. attach_session to that id.
  5. Observed: BEFORE replayed from the session buffer, AFTER streamed live post-disconnect, clean exit 0 reported.
• Reviewer to confirm against the server-side URL routing in sprites.dev.

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