feat: use oauth2 crate for OAuth with configurable client auth (#4181)

Stacked on #4173 (`jack/restore-oidc-flows`, base branch mirrored to
this repo so the diff shows only this change); context from review:
https://github.com/lancedb/lancedb/pull/4173#issuecomment-5674048100.
Rebase to `main` once #4173 and #4179 merge.

## What moved to the `oauth2` crate (5.0, no default features)

- Authorization URL generation and CSRF state (`authorize_url`,
`CsrfToken`)
- PKCE S256 challenge/verifier generation and code exchange
- Client-credentials, authorization-code, refresh-token, and device-code
grant request construction
- Device authorization request and the device token polling loop
(`authorization_pending`, `slow_down` +5s, expiry deadline, denial,
network backoff capped at 10s)
- Standard success/error response parsing (`RequestTokenError`)
- Token-endpoint client authentication and standards-compliant parameter
encoding (RFC 6749 2.3.1 Basic encoding)

LanceDB keeps ownership of OIDC discovery (compared `openidconnect`: no
measurable win for our 3-field metadata + strict validation, at real
dependency cost), HTTPS-or-loopback endpoint enforcement, the loopback
callback server, browser/stderr prompts, token caching and refresh
orchestration, and the dedicated hardened Azure IMDS source, which is
unchanged.

## Client authentication methods

New `ClientAuthMethod` enum (`none` | `client_secret_basic` |
`client_secret_post`), exposed in Rust, Python, and Node. Unset resolves
to `client_secret_basic` when a secret is present (RFC 6749 2.3.1
recommendation and the normal Okta confidential-app default, so a
default Okta app works without weakening its configuration) and to
`none` for public clients (PKCE/device). Explicit `none` with a secret,
or basic/post without one, is rejected. The method applies to client
credentials, code exchange, refresh, and device requests. Deliberate
behavior change: confidential clients previously always sent the secret
in the POST body; they now default to Basic (Keycloak accepts both).

No `audience`/`resource` parameters were added: the supported target is
an Okta custom authorization server with the API audience configured
server-side, so client-provided audience parameters are unnecessary;
`add_extra_param` support exists if a concrete provider contract ever
needs them.

## Device polling behavior changes (deliberate, tested)

- The first token poll now happens immediately rather than after one
interval (RFC 8628 allows both).
- Transient failures (HTTP 429, 5xx, `temporarily_unavailable`, network
errors) now retry with exponential backoff capped at 10s instead of
retrying at the fixed interval; polling never spins faster than once per
second even if a server reports a zero interval.

## Security and compatibility

- Issuer and discovered endpoints (and device verification URIs) still
require HTTPS except explicit loopback HTTP, enforced before any crate
URL type is built
- Token HTTP client keeps the hardened redirect policy that refuses
insecure redirect targets; regression test added
- Errors never embed raw response bodies (avoids leaking tokens through
parse failures); all credential types stay redacted in Debug
- Transient conditions (429/5xx/`temporarily_unavailable`) remain
retryable in device polling and hard errors elsewhere; refresh keeps
rotation and reauthentication semantics
- Existing public APIs stay source-compatible except the added
`OAuthConfig.client_auth_method` field

## Tests

Rust: client-auth methods across code
exchange/refresh/client-credentials/device (none/basic/post),
auth-method resolution and validation, transient device retries,
denial/expiry, redirect rejection, malformed-response leak check, PKCE
URL assertions, redaction. Python and Node: enum values, conversion,
unknown-method errors, config defaults.

Manual Okta validation recipe (no automated Okta credentials): create a
custom authorization server with an API audience, one confidential web
app (Basic) for authorization-code, one native app (PKCE, no secret),
one native device app; point `issuer_url` at the custom server, set
`client_auth_method` only for the POST-required case; verify token
acquisition, refresh after expiry, and `x-lancedb-credential-type: oidc`
against a LanceDB deployment. Never commit tenant URLs or secrets.

Co-authored-by: Xuanwo <github@xuanwo.io>

python/tests/test_oauth.py

@@ -63,6 +63,34 @@ def test_device_code_flow_value():
     assert oauth.OAuthFlowType.DEVICE_CODE.value == "device_code"
 
 
+def test_client_auth_method_values():
+    oauth = _load_oauth_module()
+
+    assert oauth.ClientAuthMethod.NONE.value == "none"
+    assert oauth.ClientAuthMethod.CLIENT_SECRET_BASIC.value == "client_secret_basic"
+    assert oauth.ClientAuthMethod.CLIENT_SECRET_POST.value == "client_secret_post"
+
+
+def test_client_auth_method_defaults_to_none():
+    oauth = _load_oauth_module()
+
+    config = oauth.OAuthConfig(
+        issuer_url="https://issuer.example.com",
+        client_id="client-id",
+        scopes=["openid"],
+        client_auth_method=oauth.ClientAuthMethod.CLIENT_SECRET_POST,
+    )
+
+    assert config.client_auth_method is oauth.ClientAuthMethod.CLIENT_SECRET_POST
+
+    default_config = oauth.OAuthConfig(
+        issuer_url="https://issuer.example.com",
+        client_id="client-id",
+        scopes=["openid"],
+    )
+    assert default_config.client_auth_method is None
+
+
 def test_token_cache_options_default_to_memory_only():
     oauth = _load_oauth_module()