[FIX] kw_api_custom_endpoint: apply the endpoint domain when change() resolves by id

[brmk]

Fixes #2.

The configured domain is applied on the LIST branch and ignored by the by-id branches.
change() resolved its target with a bare
search([(self.model_id_field, '=', obj_id)], limit=1), and Odoo only skips the implicit
active = True when a domain mentions active. An archived record was therefore never
found, write() on the empty recordset was a silent no-op, and data_response serialised
it as {"content": [], "code": "200"} — the caller told that a write succeeded when nothing
had been written.

So an endpoint configured with [("active", "in", [True, False])], which declares archived
records to be in scope, still could not write to them. The same gap let a POST update a
record the endpoint's domain was meant to exclude.

change() now resolves through a small api_get_obj_domain() helper that narrows the id
lookup by the endpoint's own domain — the same thing the list branch has always done.
No context is touched: a domain mentioning active makes Odoo skip active_test on its
own.

Behaviour change

An id that resolves to nothing now answers 400: Wrong ID — the shape response() already
uses — instead of a 200 claiming a write happened. For an endpoint with no domain, an
archived id moves from a silent fake success to that honest 400.

Verified on Odoo 19.0

Against a product.template endpoint with name changeable:

endpoint domain	target	before	after
[("active","in",[True,False])]	active	200, echoed, written	unchanged
[("active","in",[True,False])]	archived	200, content: [], not written	200, echoed, written
none	active	200, echoed, written	unchanged
none	archived	200, content: [], not written	400 Wrong ID
any	unknown id	200, content: []	400 Wrong ID

A note for reviewers

Our first attempt at this fixed it with

super(CustomEndpoint, self.with_context(active_test=False)).change(...)

That put active_test=False into the whole request environment, and kw_api's translation
helper does self.env['res.lang'].sudo().search([]). res.lang is archivable, so it began
returning inactive languages, and reading a translation for a language Odoo has not loaded
raised KeyError: '<model>.<field>' from the ORM field cache — after a successful write,
inside the response serialiser. Every write broke, not just archived ones. It reached our
production and was reverted 23 minutes later.

Applying the domain avoids that entire class of problem, which is why this PR takes that
route rather than the context one.

Left alone deliberately

response() (GET by id) and delete() share the same bare search. I did not touch them:
response() at least answers 400 honestly today, and widening what delete() can reach
seems like a maintainer's call rather than a bug fix. Both are noted in #2.