Push `environment_id` validation into `Connection.__init__`; unify cloud_provider/cloud_region validation there too

[jlrobins]

Follow-up to #210/#211, which found that connect() and Connection.__init__ disagreed on
whether providing both endpoint and (cloud_provider, cloud_region) was an error -- fixed by
moving that check down into __init__ so both connect() and direct Connection(...)
construction behave identically. Auditing the rest of connect()'s validation for the same
disease turned up two more spots.

1. environment_id is only validated in connect(), not __init__

connect() raises:

if not environment_id:
    raise InterfaceError("Environment ID is required")

Connection.__init__ has no equivalent check -- it just does self.environment_id = environment_id and moves on. Confirmed by hand that this constructs without error today:

Connection(
    flink_api_key="k", flink_api_secret="s",
    environment_id="", organization_id="org-id",
    cloud_provider="aws", cloud_region="us-east-1",
    endpoint=None,
)

environment_id gets interpolated directly into request paths (e.g.
f"/environments/{self.environment_id}"), so an empty value doesn't fail fast -- it produces a
malformed URL that only surfaces later as a confusing server-side 404 / OperationalError
instead of a clear InterfaceError at construction time.

Unlike organization_id (see below), there's no comment or docstring anywhere suggesting this
asymmetry is intentional. It looks like a plain oversight.

Fix: move the environment_id truthiness check down into Connection.__init__, the same way
the endpoint/cloud-info check was moved in #211, so connect() and direct Connection()
construction both fail fast with the same error.

(Note: organization_id's current connect()-only enforcement is not in scope here -- its
docstring in __init__ explicitly documents that the constructor intentionally leaves it empty
when no global key is present, deferring to connect(), because a global key enables lazy
inference later via the organization_id property. Pushing that down would require porting the
same "unless a global key is provided" conditional into __init__, which is a real design
decision rather than a straightforward bug fix -- worth its own separate discussion if we want to
tackle it.)

2. Unify the cloud_provider/cloud_region "missing" check to live only in __init__

Both connect() and __init__ already raise when endpoint is absent and cloud_provider
and/or cloud_region is also missing, so there's no behavioral gap here -- but the two raise
sites duplicate the same validation with different wording:

• connect() raises two distinct, field-specific messages:
  • "Cloud provider is required when endpoint is not provided"
  • "Cloud region is required when endpoint is not provided"
• Connection.__init__ raises one combined message:
  • "cloud_provider and cloud_region are required when endpoint is not provided"

Tests currently pin both wordings independently (test_connection_unit.py, around the
test_requires_cloud_provider / test_requires_cloud_region tests using connection_factory,
and the direct-Connection() construction test using the combined message).

Fix: collapse this into a single check living only in __init__ (removing the duplicate from
connect()), so there's one source of truth. Prefer keeping the more specific per-field
wording (nicer error messages), so __init__ would raise
"Cloud provider is required when endpoint is not provided" /
"Cloud region is required when endpoint is not provided" as appropriate. Tests that currently
pin the combined __init__-specific message will need updating to match.

Scope

Both are InterfaceError message/placement changes only, no behavioral change for connect()
callers (who already got these errors) -- the only newly-affected callers are those constructing
Connection directly.