Add basic-auth (username/password) login for Dremio Software deployments

[cq2wgwtzb5-lgtm]

Closes #125

What

Adds an optional dremio.basic_auth configuration block for Dremio Software deployments that cannot issue PATs (e.g. Community edition — the PAT support key is absent and token endpoints 404 there):

dremio:
  uri: https://your-dremio-instance:9047
  basic_auth:
    username: alice
    password: "@~/tokens/dremio.password"  # or a direct value

How

• config/settings.py — new BasicAuth model mirroring OAuth2 (username, password with the existing @-file resolution convention, expiry tracking via has_expired); Dremio.basic_auth field + basic_auth_configured property (guarded off for Cloud).
• api/basic_auth.py (new) — exchanges credentials for a session token via POST /apiv2/login; SessionToken.update_settings() installs it as the effective token, with a 5-minute safety margin ahead of the server-reported expiry. Stdlib urllib only, no new dependencies.
• api/transport.py — one branch in DremioAsyncHttpClient.__init__, exactly parallel to the OAuth branch: if basic auth is configured and there is no valid token (or the session expired), log in and refresh. No changes to header construction — the REST API accepts session tokens as Bearer tokens.

PAT and Dremio Cloud behavior are unchanged; if both pat and basic_auth are configured, the PAT wins.

Testing

• 5 new unit tests (tests/api/test_basic_auth.py): settings parsing, @-file password resolution + serialization keeping the reference (not the resolved secret), Cloud guard, mocked login flow updating settings, expiry detection.
• Full suite: 353 passed, 44 skipped; black clean.
• Live verification: dremio-mcp-server tools invoke -t RunSqlQuery end-to-end against a real Dremio Software Community deployment using only username/password.

Docs

docs/settings.md gains a "Basic auth (Dremio Software without PAT support)" section.

🤖 Generated with Claude Code

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