fix: fix redash dashboard exporter

[gjxdxh]

Summary of Changes

Fix issues in redash dashboard exporter and added a few tests

Tests

modified unit test

Documentation

What documentation did you add or modify and why? Add any relevant links then remove this line

CheckList

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[feng-tao]

@gjxdxh could you take a look at the CI error which fails mypy:

flake8 .
mypy .
databuilder/extractor/dashboard/redash/redash_dashboard_extractor.py:175: error: Argument 1 to "DashboardChart" has incompatible type "**Dict[str, object]"; expected "Optional[str]"
databuilder/extractor/dashboard/redash/redash_dashboard_extractor.py:175: error: Argument 1 to "DashboardChart" has incompatible type "**Dict[str, object]"; expected "str"
tests/unit/extractor/dashboard/redash/test_redash_dashboard_extractor.py:135: error: Argument "query_id" to "DashboardQuery" has incompatible type "int"; expected "Optional[str]"
tests/unit/extractor/dashboard/redash/test_redash_dashboard_extractor.py:146: error: Argument "query_id" to "DashboardChart" has incompatible type "int"; expected "str"
tests/unit/extractor/dashboard/redash/test_redash_dashboard_extractor.py:147: error: Argument "chart_id" to "DashboardChart" has incompatible type "int"; expected "str"
tests/unit/extractor/dashboard/redash/test_redash_dashboard_extractor.py:156: error: Incompatible types in assignment (expression has type "DashboardTable", variable has type "DashboardChart")
Found 6 errors in 2 files (checked 261 source files)
Makefile:15: recipe for target 'mypy' failed
make: *** [mypy] Error 1
Error: Process completed with exit code 2.