Add Tasks API endpoint tests for DAG without start_date

[omkar-foss]

related: #40828

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This PR adds tests to cover Tasks API endpoint cases where the DAG doesn't have a start_date and is defined with schedule=None (it's unscheduled). Tests in this PR support the fix in #40878.

Additionally, this PR fixes the Provider KeyError that occurs in breeze selective checks for API tests as mentioned in the comment below. Update: This is fixed in #40904.

[boring-cyborg]

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[omkar-foss]

Build is failing due to a KeyError in breeze selective checks here. But this PR has only API-related tests (no Provider-related code changes).

Could this be a bug @potiuk? The breeze selective checks code is breaking here in this build, as candidate_test_types holds values API and Always so removing Providers throws the KeyError.

Update: I've fixed this KeyError issue in this PR so it can be tested and verified here itself. Let me know if I should move it to a PR of it's own. Thanks.

[potiuk]

I separated out the fix in Breeze @omkar-foss (and did it slightly differently and added tests covering more cases). #40904

[potiuk]

( I made you co-author)

[potiuk]

You can rebase on top of main and remove your changes - it should work now @omkar-foss

[omkar-foss]

Thanks for looking into this @potiuk. Done, I've rebased this PR with main and have removed the selective checks fix changes from this PR. Build is passing here, so #40904 looks to be working well ✅

@jedcunningham this is a tests-only PR supporting the change in your PR #40878, please have a look whenever you get a minute, thanks.

[boring-cyborg]

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