[Jobs] Support --num-jobs without a pool

[Michaelvll]

Summary

sky jobs launch --num-jobs N was restricted to pool submissions: the CLI, client SDK, and server each raised an error if --num-jobs was given without --pool. This PR lifts that restriction so --num-jobs N works without a pool, submitting N independent managed jobs (each on its own cluster).

The submission machinery was already pool-agnostic:

• The local (consolidation) and remote submission paths both plumb num_jobs through with pool_hash=None.
• SKYPILOT_NUM_JOBS and the per-job SKYPILOT_JOB_RANK env vars are set for all jobs regardless of pool.
• Concurrency is already throttled by the jobs scheduler's default path (get_number_of_jobs_controllers()), so without a pool the jobs queue as PENDING and start as controller capacity frees — the same behavior as launching N jobs individually.

So the only change needed was removing the three guards, plus two output fixes the guards were masking:

• The launch confirmation prompt now reports the job count regardless of pool (previously only set in the pool branch).
• The "Jobs submitted" summary no longer emits pool-only hints when there is no pool — previously it showed a dashboard link filtered by value=None and sky jobs cancel --pool None. Without a pool it now links to the unfiltered jobs page and suggests sky jobs cancel <job-ids>.
• A short notice is printed for the non-pool multi-job case.

Changes

• sky/client/cli/command.py: remove the CLI guard; branch the multi-job summary hints on pool presence; add a non-pool multi-job notice.
• sky/jobs/client/sdk.py: remove the client-side guard; report job count in the confirmation prompt regardless of pool.
• sky/jobs/server/core.py: remove the server-side guard.
• tests/unit_tests/test_sky/test_cli_launch_validation.py: tests that --num-jobs without --pool is accepted and reaches launch with pool=None, and that the multi-job output contains no pool-specific hints when there is no pool.

Test plan

• pytest tests/unit_tests/test_sky/test_cli_launch_validation.py — passes.
• Manual: sky jobs launch --num-jobs 3 'echo hi' (no --pool) against a remote API server creates 3 managed jobs, each on its own cluster, with distinct SKYPILOT_JOB_RANK (0–2) and SKYPILOT_NUM_JOBS=3; the summary shows the unfiltered "Show all jobs" link and a sky jobs cancel <ids> hint.

[gemini-code-assist]

Code Review

This pull request allows specifying --num-jobs without --pool when launching managed jobs, enabling users to submit multiple independent jobs on their own clusters. It updates the CLI messages, dashboard hints, and cancel hints to correctly handle cases where a pool is not specified, and adds corresponding unit tests. The feedback suggests handling the singular form correctly when num_jobs is explicitly set to 1 to avoid grammatically incorrect output like "1 managed jobs".

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[gemini-code-assist]

When --num-jobs 1 is explicitly passed, the confirmation prompt will display '1 managed jobs', which is grammatically incorrect. Checking if num_jobs is None or 1 ensures the prompt correctly uses the singular form 'a managed job'.

Suggested change

	job_identity = ('a managed job'
	if num_jobs is None else f'{num_jobs} managed jobs')
	job_identity = ('a managed job'
	if num_jobs is None or num_jobs == 1 else f'{num_jobs} managed jobs')