feat(fal_client): add a tags request option

[dazip]

Why: callers want to attach their own key=value tags to fal requests so usage and cost can be attributed to their own dimensions (team, environment, feature). The gateway already parses and stamps the packed X-Fal-Tags header; the Python client had no way to send it.

Adds a tags= option to run, submit, subscribe, and stream on both SyncClient and AsyncClient (and therefore the module-level fal_client.run/submit/... aliases). Tags are packed into a single X-Fal-Tags: k=v,k=v header by a new add_tags_header helper in _headers.py, which validates them the way the gateway does — keys lowercased and limited to [a-z0-9._-], values printable ASCII without commas, at most 10 pairs, 64-char keys, 256-char values, 1 KB of keys and values, and the reserved fal.* namespace refused. The gateway fails open and silently drops pairs it rejects, so the client raises ValueError up front instead.

---

Note

Low Risk
Additive optional parameter and request-header validation only; no changes to auth, billing logic, or existing call paths when tags is omitted.

Overview
Adds optional tags= on run, submit, subscribe, and stream for SyncClient and AsyncClient, so callers can attach custom dimensions (team, env, feature) for usage attribution.

Tags are validated client-side in new add_tags_header (limits and rules aligned with the gateway: key format, printable ASCII values, max pairs/lengths, 1 KB budget, no fal.* keys), then sent as a single X-Fal-Tags: key=value,... header. Invalid tags raise ValueError before any HTTP call, instead of being silently dropped server-side.

Unit tests cover header packing/validation and that tagged requests set the header on sync/async paths (including subscribe forwarding to submit).

^{Reviewed by Cursor Bugbot for commit 4cbc0a3. Bugbot is set up for automated code reviews on this repo. Configure here.}

[linear-code]

PROD-4145

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.