feat(python-wrapper): use explicit parameters in Client.__init__

[caoergou]

Change Description

Background

Builds on #10284 (mypy CI integration) to improve the Python wrapper's type safety and usability.
`Client.init` previously accepted only `**kwargs`, giving IDEs no help with auto-completion or
type checking, and leaving users to guess what parameters are valid.

Changes

`Client.init` — explicit parameters

The six most common configuration parameters are now named keyword-only arguments:

```python

Before

client = Client(host="...", username="...", password="...") # no IDE hints

After — IDE shows completions, mypy checks types

client = Client(host="...", username="...", password="...")
```

Parameters added: `host`, `username`, `password`, `access_token`, `verify_ssl`, `proxy`.
`**kwargs` is retained so less common `lakefs_sdk.Configuration` options (e.g. `ssl_ca_cert`) keep working.

Note: only non-`None` values are forwarded to `ClientConfig` to preserve its auto-detection behaviour
(env vars / `~/.lakectl.yaml`) when no credentials are supplied.

Return type annotations

Added return types to previously unannotated members:

Member	Return type
`config`	`ClientConfig`
`sdk_client`	`LakeFSClient`
`storage_config`	`ServerStorageConfiguration`
`reset_time`	`Optional[datetime.datetime]`
`storage_config_by_id` (Client + ServerConfiguration)	`ServerStorageConfiguration`
`_refresh_token_if_necessary`	`None`

Class attribute annotations

`_client` and `_conf` are now declared as non-Optional at the class level (`LakeFSClient` / `ClientConfig`),
since `init` always sets them unconditionally. `_server_conf` remains `Optional` (lazy-loaded).

IAM token refresh — None guards

Annotating `_refresh_token_if_necessary` caused mypy to check its body, exposing latent issues:

• `iam_provider` (type `Optional[IAMProvider]`) was accessed without a None check
• `iam_provider.aws_iam` (type `Optional[AWSIAMProviderConfig]`) was accessed without a None check
• `urlparse(host).hostname` returns `Optional[str]`, passed to a `str` parameter

These are now guarded with explicit `if` checks (not `assert`, which is disabled under `python -O`).
In `init`, an invalid hostname raises `ValueError` with a clear message. In the token-refresh path,
an invalid hostname silently skips the refresh to avoid surfacing a hard error during an in-flight call.

Testing Details

• `bash lint.sh` — mypy clean (0 errors, 20 files); pylint 10.0/10 on changed file
• `pytest tests/utests/` — 53 passed

Closes #8720