feat: implement the URL search params serialization standard

Port @seamapi/url-search-params-serializer to Python so the SDK can
serialize objects to URL search params for HTTP GET requests.

Output is byte-for-byte identical to the reference implementation:

- Values are encoded with the application/x-www-form-urlencoded
  serializer, which differs from urllib in its treatment of "*" and "~".
- Params are sorted by name, compared by UTF-16 code unit.
- Floats are formatted using the ECMAScript Number::toString algorithm,
  which differs from repr for integral floats and around the exponent
  notation thresholds.

Python has no undefined, so UNDEFINED is provided as the sentinel for a
removed param, while None serializes to an empty value as null does.
Temporal.Instant and Date both map to datetime, where a naive datetime is
interpreted as UTC and microseconds are truncated to millisecond
precision.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PpqwDhZmyikGbjmCPqFW2A

Author: claude

README.rst

@@ -65,6 +65,8 @@ Contents
 
     * `Setting the endpoint`_
 
+    * `Serializing URL search params`_
+
 * `Development and Testing`_
 
   * `Quickstart`_
@@ -436,6 +438,49 @@ e.g., testing or proxy setups.
 
 Either pass the ``endpoint`` option to the constructor, or set the ``SEAM_ENDPOINT`` environment variable.
 
+Serializing URL search params
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The Seam API parses URL search params as complex types.
+This SDK implements the `Seam URL search params serialization standard`_,
+which defines how the Seam SDKs serialize objects to URL search params.
+Use it directly when building requests to the Seam API by hand:
+
+.. code-block:: python
+
+  from seam import serialize_url_search_params
+
+  serialize_url_search_params(
+      {
+          "name": "Dax",
+          "age": 27,
+          "is_admin": True,
+          "tags": ["cars", "planes"],
+      }
+  )
+  # => 'age=27&is_admin=true&name=Dax&tags=cars&tags=planes'
+
+Params are sorted by name, so equivalent input always produces the same query string.
+Nested dicts are serialized to dot-path keys, e.g., ``{"a": {"b": 1}}`` becomes ``a.b=1``.
+Params set to ``None`` are serialized to an empty value, e.g., ``a=``,
+while params set to ``seam.UNDEFINED`` are removed.
+A param that cannot be represented raises a ``seam.UnserializableParamError``.
+
+To merge serialized params into existing params, use ``update_url_search_params``:
+
+.. code-block:: python
+
+  from seam import UrlSearchParams, update_url_search_params
+
+  search_params = UrlSearchParams("?foo=bar")
+
+  update_url_search_params(search_params, {"name": "Dax"})
+
+  str(search_params)
+  # => 'foo=bar&name=Dax'
+
+.. _Seam URL search params serialization standard: https://github.com/seamapi/url-search-params-serializer
+
 Development and Testing
 -----------------------
 

seam/__init__.py

@@ -15,3 +15,10 @@
 )
 from .seam_webhook import SeamWebhook
 from svix.webhooks import WebhookVerificationError as SeamWebhookVerificationError
+from .utils.url_search_params_serializer import (
+    UNDEFINED,
+    UnserializableParamError,
+    UrlSearchParams,
+    serialize_url_search_params,
+    update_url_search_params,
+)