sync: replace hardcoded concurrency limit with --concurrency flag

[luketchang]

The sync engine capped in-flight requests at a hardcoded MaxRequestsInFlight = 20, applied to file PUTs, deletes, mkdirs and rmdirs alike. That ceiling is fine for most workspaces but is too conservative for fast networks / high-tier workspaces (slow uploads of larger app bundles) and too aggressive when the workspace is rate-limiting (we want to dial it down without recompiling).

This change:

• Adds Concurrency to libs/sync.SyncOptions, normalised to MaxRequestsInFlight in sync.New when unset so existing callers don't need updating.
• Threads the limit through groupRunParallel/applyDiff in libs/sync/watchdog.go; the constant is kept as the documented default.
• Surfaces a --concurrency int flag on the two user-facing entry points that drive this code path: 'databricks sync' and 'databricks bundle sync' (the latter is what backs the apps deploy/sync flow).
• Validates --concurrency >= 1 at flag-parse time per the project rule on rejecting incompatible inputs early.
• Adds TestGroupRunParallelHonorsLimit to lock in the parallelism cap.

Slack thread:
https://replit.slack.com/archives/C0A2Z9042FR/p1778082184841149

Changes

Why

Tests

[github-actions]

Approval status: pending

/cmd/bundle/ - needs approval

Files: cmd/bundle/sync.go
Suggested: @pietern
Also eligible: @denik, @shreyas-goenka, @anton-107, @andrewnester, @janniklasrose, @lennartkats-db

/cmd/sync/ - needs approval

Files: cmd/sync/sync.go
Suggested: @simonfaltum
Also eligible: @renaudhartert-db, @hectorcast-db, @parthban-db, @tanmay-db, @Divyansh-db, @tejaskochar-db, @mihaimitrea-db, @chrisst, @rauchy

/libs/sync/ - needs approval

Files: libs/sync/sync.go, libs/sync/watchdog.go, libs/sync/watchdog_test.go
Suggested: @simonfaltum
Also eligible: @renaudhartert-db, @hectorcast-db, @parthban-db, @tanmay-db, @Divyansh-db, @tejaskochar-db, @mihaimitrea-db, @chrisst, @rauchy

_{Any maintainer (@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka, @simonfaltum, @renaudhartert-db) can approve all areas.
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[github-actions]

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• PR number: 5196
• Commit SHA: c50e7521a76d0968e6f1a52b35a5aea2e96ce78b

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