Migrate Exasol provider to pyexasol 2.x

[Rishabjain999]

pyexasol 2.x ships a py.typed marker that exposed 8 type mismatches in
the Exasol hook, causing CI to cap the dependency at <2. This PR removes
the cap by fixing the hook to be compatible with pyexasol 2.x.

Changes:

• Rename reused conn variable in get_conn() to airflow_conn / exa_conn
  to eliminate the mypy type-narrowing conflict.
• Add _validate_query_params() static method to narrow
  Iterable|Mapping|None → dict|None, matching pyexasol 2.x's
  execute() and export_to_pandas() signatures. Raises TypeError
  with a clear message for non-dict input.
• Narrow sql: str | list[str] → sql: str in get_records() and
  get_first(), matching what pyexasol actually accepts.
• Bump pyexasol dependency from >=0.26.0,<2 to >=2.0.0,<3.
• Update existing test to use dict parameters (tuples were never valid).
• Add negative tests for non-dict parameters and list SQL inputs.

closes: #69123

[boring-cyborg]

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[eladkal]

We already have several PRs for that issue. We don't need more. Please help review the previously submitted PRs

[Rishabjain999]

@eladkal any other pr or issue I can work on?