Fix invalid AuthMech crashing with Recursive update / StackOverflowError

[sreekanth-db]

Summary

• Connecting with an unsupported AuthMech (e.g. AuthMech=99) intermittently failed with an internal IllegalStateException: Recursive update or a StackOverflowError on both the SEA and Thrift paths, instead of a clean validation error.
• Root cause: AuthMech.parseAuthMech throws during ClientConfigurator construction, which runs inside DatabricksClientConfiguratorManager.getConfigurator's ConcurrentHashMap.computeIfAbsent. The exception's constructor emits a telemetry failure log, whose feature-flag refresh re-enters getConfigurator for the same connectionUuid → a reentrant computeIfAbsent → Recursive update (or, depending on timing, StackOverflowError).
• Fix: validate AuthMech at connect time in DatabricksConnectionContext.parse() (via ValidationUtil.validateInputProperties → new validateAuthMech), before the configurator machinery runs. Unsupported values are now rejected deterministically with a DatabricksValidationException (SQLState=INPUT_VALIDATION_ERROR).
• A pure, side-effect-free AuthMech.fromValue(int) lookup is introduced as the single source of truth for supported values (3=PAT, 11=OAuth); parseAuthMech delegates to it, so adding a new AuthMech is a one-line change. parseAuthMech's behavior, exception type, and messages are unchanged.

Test plan

• New unit tests ValidationUtilTest#testValidateAuthMech_*: supported values (3, 11, omitted) pass; unsupported/non-integer/empty throw INPUT_VALIDATION_ERROR.
• New unit test DatabricksConnectionContextTest#parse_rejectsUnsupportedAuthMech_withInputValidationError.
• Existing AuthFlowParsing and full ValidationUtilTest (47) / DatabricksConnectionContextTest (145) suites pass.
• Manual repro against a live warehouse: AuthMech=99 went from IllegalStateException: Recursive update (5/5 attempts on both SEA and Thrift) to a clean DatabricksValidationException / INPUT_VALIDATION_ERROR (5/5 on both transports).