Clarify what is_active means for a file-service connection

[martsokha]

Context

WorkspaceConnection.is_active gates connection actions: it's checked before manual sync, import, export, picker-token, and scheduled runs (e.g. connection_syncs.rs, connections.rs, the sync worker). For an object store this is clearly useful — set is_active = false to pause scheduled sync while keeping the config and credentials.

For a file-service connection (Google Drive, Dropbox, OneDrive, Box), the value is much weaker:

• File services have no scheduled runs (their schedule row carries no cron; the scheduler only lists connections with schedule_cron IS NOT NULL). So "disable to pause automatic sync" — the main reason to disable an object store — does not apply.
• Every file-service action is user-initiated (picker import, per-file export, picker-token). Disabling only blocks the user's own on-demand actions, which is nearly the same as "don't use it."
• Disabling does not revoke or affect the stored OAuth tokens; token refresh happens only on use.

The question

What should is_active = false mean for a file-service connection? Options considered:

1. Leave it as a uniform field (current). Harmless; still has a defensible admin use — a workspace admin turns a connection off for all members (blocks import/export through it) without deleting the OAuth grant. Consistent API across connection types.
2. Ignore/hide it for file services. Stop gating on-demand file-service actions on is_active (or omit the toggle for file services), since it only blocks the user's own actions. Makes the field's meaning provider-dependent — arguably worse for API consistency.
3. Give it real meaning. Make disabling actually do something distinct — refuse token refresh, mark the connection for reconnect, etc. More machinery for an unclear benefit.

Leaning toward (1): keep it uniform, and treat it as an admin-level "turn this connection off for everyone without losing tokens" switch — which is a real (if minor) use even for file services. This issue is to decide and document the intended semantics rather than leave it implicit.

Not blocking

No correctness bug — the current behavior (gate all actions on is_active) is safe. This is a semantics/clarity decision. Related: #277 (connectors), #282 (picker token + export).