Add --synthetic_duration option for time-based synthetic workloads

[zeke-emerson]

Problem

Running synthetic benchmarks for a specific wall-clock duration currently requires calculating the right --synthetic_requests value (desired_duration * requests_per_sec). This is unintuitive — users just want to say "run for 5 minutes at 10 QPS" without doing math.

Solution

Add a --synthetic_duration CLI option (in seconds) as an alternative to --synthetic_requests. When specified, query iterators yield indefinitely and RunUser checks a time-based deadline to end the Run phase. The progress bar shows elapsed/total time instead of request counts.

Key design decisions:

• Mutually exclusive with --synthetic_requests — specifying both produces an error
• Backward compatible — when neither is specified, defaults to --synthetic_requests=100
• Scoped to synthetic and synthetic-proportional — not supported with synthetic-runbook (its multi-step structure makes duration splitting complex; can be added later)
• Deadline check in RunUser — keeps timing logic in one place rather than threading it through every iterator

Files changed:

• vsb/cmdline_args.py — new argument + validation
• vsb/workloads/base.py — synthetic_duration() method on base class
• vsb/workloads/synthetic_workload/synthetic_workload.py — duration mode for InMemoryWorkload, SyntheticWorkload, and SyntheticProportionalWorkload
• vsb/users.py — deadline check in RunUser.do_run(), time-based progress bar in LoadShape

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)

Test Plan

• All 33 existing unit tests pass (poetry run pytest tests/unit)
• Verified --synthetic_duration=30 --requests_per_sec=10 runs for ~30 seconds against Pinecone
• Verified omitting both flags defaults to --synthetic_requests=100 (backward compat)
• Verified specifying both --synthetic_duration and --synthetic_requests produces an error
• Verified --synthetic_duration with synthetic-runbook produces an error

[dmsmith25]

Tested locally with and without the --synthetic_duration tag. Both tests succeeded.