feat: apply the URL search params standard and type nullable params

[razor-x]

Hooks the serializer added in #611 into the client, so requests actually go
out with the standard applied, and consumes the blueprint isNullable flag so
NULL is typed precisely. No method changes: routes keep using
preferredMethod exactly as they do today.

Serializing request params

SeamHttpClient.request now does two things before handing off to httpx:

• Any mapping passed as params is run through
  serialize_url_search_params, and the resulting query string is set on the
  url with httpx.URL(url, query=query.encode()). httpx never sees the
  params, so it never re-encodes them.
• NULL sentinels in a json body are replaced with None, via a new
  replace_null in seam/null.py, so json.dumps emits them as null.
  Without it a body carrying the sentinel raises
  TypeError: Object of type Null is not JSON serializable.

Params that are not a mapping — an already-built query string, or the pairs
of a UrlSearchParams — are passed through to httpx untouched, so a caller
who serialized them already keeps control of the representation.

Why the query is set on the url

httpx would otherwise re-encode what the serializer produced. The two
disagree on arrays and nested objects, and on exactly two characters:

params	httpx	this PR
{"device_ids": []}	?	device_ids=
{"custom_metadata_has": {"k": "v"}}	custom_metadata_has=%7B%27k%27%3A+%27v%27%7D	custom_metadata_has.k=v
{"search": "a *~ b"}	search=a+%2A~+b	search=a+*%7E+b

The character differences are cosmetic — the API decodes both to the same
pairs — but the array and nested-object rows are not: the empty array
currently disappears from the query entirely, and a nested object currently
goes out as the repr of a Python dict.

The query survives verbatim, including through relative-url merging against
base_url, because httpx carries it in raw_path.

Typing nullable params

nullableType renders a param the API documents as nullable as
Union[<type>, Null], so it accepts the sentinel:

seam.devices.update(device_id="...", name=NULL)       # name is nullable
seam.devices.update(device_id="...", is_managed=NULL) # fails the type check

A param that is merely optional deliberately does not accept it. Optional
params are omitted by passing None, where sending null would unset a value
instead, so accepting the sentinel everywhere would invite exactly the mistake
it exists to prevent.

This covers 49 params across 20 route modules — 43 Union[str, Null], 4
Union[float, Null], 2 Union[int, Null] — and the from ..null import Null
import is emitted only for modules that need it.

Scope

Under preferredMethod a route only gets GET/DELETE when all of its
params are simple, since getPreferredMethod falls back to POST for
complex ones. So generated routes exercise the simple path, and the array and
nested-object paths are reached through direct seam.client.get(...) calls.

Tests

test/search_params_test.py asserts on the exact bytes the SDK puts on the
wire via the recording server: standard serialization, no re-encoding, None
omitted, NULL sent empty, no ? when there are no params, every verb,
non-mapping pass-through, UnserializableParamError raised before the
request, NULL in a JSON body, and a generated route.

test/nullable_param_test.py covers the three states end to end — NULL sent
as null, None omitted, a real value sent — on both a nullable string and a
nullable number. Because mypy runs over test/ in CI, these also fail the
Lint job if a nullable param ever loses its Null type.

The recording server now records method, the raw target, and path/query
split apart, and answers every verb rather than just GET and POST —
/devices/update is PATCH, which the old handler answered with a 501.

159 passing, up from 141 on beta. pylint 10.00/10, black, mypy,
rstcheck, tsc, eslint, and prettier clean; npm run generate is
idempotent.

The end-to-end round trip through the API's parser — e.g.
seam.client.get("/devices/list", params={"device_ids": []}) returning zero
devices — still needs the @seamapi/fake-seam-connect 2.x bump, since the
pinned 1.86.0 has no parser and rejects device_ids=. That bump belongs with
the semanticMethod change, where it is required.

README

Adds a Setting a Param to Null section covering the omit-versus-null
distinction, None versus NULL, why null is never the default, Null as
the exported type, and which params accept the sentinel.