Add no-arg close_idle_connections! + document form-body decoding (#1118, #1123, #1137)

[quinnj]

Resolves #1118, #1123, #1137 — three related triage outcomes from the same area, bundled.

#1137 — closing cached connections

The 1.x complaint was that closeall emptied the pool without closing the connections. In 2.0 close_idle_connections!(transport) already closes them — but it required a Transport you own, with no way to reach the default pool used by HTTP.get/HTTP.post/etc.

Adds two overloads:

• HTTP.close_idle_connections!() — closes the default client's idle connections (no-op if no request has been made yet)
• HTTP.close_idle_connections!(client) — closes a specific client's pool

and expands the docstring to document all three forms.

#1118 / #1123 — decoding form bodies / getting POST params

These asked for a way to decode application/x-www-form-urlencoded bodies (#1118) and read POST params easily (#1123). The capability already exists: HTTP.queryparams decodes + (and %20) as space, so HTTP.queryparams(String(req.body)) parses a form body and round-trips the Dict/NamedTuple encoding HTTP.post produces. The gap was discoverability — this adds a "Reading POST form parameters on the server" example to the client guide.

Tests

• close_idle_connections!() / (client) clear the pool (idle count → 0).
• queryparams(String(_form_urlencode(dict))) == dict round-trip, incl. + and %20.
• http_forms_tests 134/134 and http_client_transport_tests 90/90 locally.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.73%. Comparing base (192978d) to head (fa2cee9).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1279      +/-   ##
==========================================
+ Coverage   84.57%   84.73%   +0.16%     
==========================================
  Files          28       28              
  Lines       10759    10767       +8     
==========================================
+ Hits         9099     9123      +24     
+ Misses       1660     1644      -16     

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