chore(tests): move integration tests to a separate crate

[michalrus]

Context

In preparation for the upcoming end-to-end (E2E) tests between the Gateway and the Platform.

I'd like this second upcoming PR to have a smaller diff that just adds new tests.

We weren’t running Gateway tests!

After the subtree merge of Gateway into this monorepo in:

• Add blockfrost-gateway to this monorepo #464

… it turns out we stopped running the Gateway tests, because their file name did not include the _test.rs suffix!

These tests were also failing after the update of Axum from 0.7 to 0.8, which changed the path segment capture format, see https://github.com/blockfrost/blockfrost-platform/actions/runs/23552254822/job/68570410736:

thread 'test_websocket_request_response_flow' (418964) panicked at crates/integration_tests/src/gateway/mod.rs:13:10:
Path segments must not start with `:`. For capture groups, use `{capture}`. If you meant to literally match a segment starting with a colon, call `without_v07_checks` on the router.
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

These issues have also been corrected by this PR.

Now, we’re running all tests under crates/integration-tests/, regardless of their name.

Review

Since there's been white space changes coupled with file renames, it’s better to review the first two commits first, and then just the last commit with "hide white space" enabled:

1. main...6e60c82
2. 6e60c82...pull/497/head

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