feat: add standby_request for calling Standby Actors, with per-request compression in HTTP clients

[vdusek]

Adds ActorClient.standby_request() (and the async twin) for sending HTTP requests to an Actor in Standby mode, so users don't have to look up standby_url and attach the token by hand.

response = client.actor('username/actor-name').standby_request('/search', params={'q': 'apify'})

The design follows the CLI RFC (apify/apify-cli#1299):

• The caller passes a path, never a URL. The host always comes from the Actor's standbyUrl returned by the API, so the token can't be sent to another host. Both built-in HTTP clients drop Authorization on a cross-host redirect.
• The token goes in the Authorization header. The Standby router removes it before forwarding, so it never reaches the Actor's code. An Actor that reads Authorization for its own auth gets the Apify token in X-Apify-Authorization instead, as described in the platform docs.
• The standbyUrl is fetched on the first call and reused by the same client.
• Request bodies are sent uncompressed by default, since Standby servers may not decode them. The compression argument opts in, with an algorithm name or an HttpCompressor.
• The timeout defaults to long, since the first request to an idle Actor waits for a Standby run to start.
• Error statuses raise ApifyApiError, and 5xx/429 are retried like API calls. The docs say so, because a non-idempotent endpoint may get the request twice.

To support the opt-in, HttpClient.call and HttpClientAsync.call take a per-request compression argument: True uses the client's compressor, an HttpCompressor is used for that request only, and False sends the body uncompressed. A custom HTTP client that replaces call itself (not just send_request) needs to accept the new argument to work with standby_request.

Also fixes the async docstring sync script, which turned ApifyClientError into the nonexistent ApifyClientAsyncError.

Naming keeps standby, as agreed in #product-server-actors until the Server Actors rename reaches the API. Includes a concepts page covering authentication, compression, timeouts, and retries. JS parity: apify/apify-client-js#1159.

Open question: keep standby (it matches the current API fields standbyUrl and actorStandby), switch to "server" to follow the Server Actors rename, or keep standby and mark the method experimental so it can be renamed later?

Closes: #973

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.56%. Comparing base (304c8e9) to head (b3995a6).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1153      +/-   ##
==========================================
+ Coverage   95.53%   95.56%   +0.03%     
==========================================
  Files          61       61              
  Lines        5974     5995      +21     
==========================================
+ Hits         5707     5729      +22     
+ Misses        267      266       -1     

Flag	Coverage Δ
integration	90.82% <41.66%> (+0.19%)	⬆️
unit	89.84% <100.00%> (+0.82%)	⬆️

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