tests: vault_config_provider flakes

Seeing the vault_config_provider integration test failing recently, but seemingly not for every run.

Sample failure from a recent 'all-integration-tests' run:

Error: Expected status 200 for / but got 500
test integration_tests::test_vault_config_provider ... FAILED
Body: ''
Stderr: 'Preparing Wasm modules is taking a few seconds...

2025-04-01T21:42:13.786690Z ERROR rustify::client: error=Server returned error
2025-04-01T21:42:13.786717Z ERROR rustify::endpoint: error=Server returned error
2025-04-01T21:42:13.786735Z ERROR vaultrs::api: error=An error occurred with the request

thread '<unnamed>' panicked at integration-variables/src/lib.rs:8:47:
Failed to acquire password from vault: Error::Provider("Failed to check Vault for config")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
2025-04-01T21:42:13.788848Z ERROR spin_trigger_http::server: Error processing request: guest invocation failed

Caused by:
    0: error while executing at wasm backtrace:
           0: 0x969b4 - vault_variables_test.wasm!__rust_start_panic
           1: 0x96906 - vault_variables_test.wasm!rust_panic
           2: 0x9681b - vault_variables_test.wasm!std::panicking::rust_panic_with_hook::he1beb51ce54153b5
           3: 0x959bc - vault_variables_test.wasm!std::panicking::begin_panic_handler::{{closure}}::hdbff82bd9d56f6f9
           4: 0x95928 - vault_variables_test.wasm!std::sys::backtrace::__rust_end_short_backtrace::h5bb26fcae04a79f5
           5: 0x961af - vault_variables_test.wasm!rust_begin_unwind
           6: 0xa4938 - vault_variables_test.wasm!core::panicking::panic_fmt::h2781e55e5f70e742
           7: 0xa62bc - vault_variables_test.wasm!core::result::unwrap_failed::he956e17bd675892a
           8: 0xb744 - vault_variables_test.wasm!core::result::Result<T,E>::expect::hefac314eb8947b31
           9: 0xf862 - vault_variables_test.wasm!vault_variables_test::config_test::h28c6d2399c7bb6c9
          10: 0xd926 - vault_variables_test.wasm!vault_variables_test::__spin_wasi_http::<impl vault_variables_test::__spin_wasi_http::preamble::exports::wasi::http::incoming_handler::Guest for vault_variables_test::__spin_wasi_http::preamble::Spin>::handle::{{closure}}::hfd0cc454113668d2
          11: 0x422a - vault_variables_test.wasm!spin_sdk::http::executor::run::hab96facdf3aba85d
          12: 0xa9a2 - vault_variables_test.wasm!vault_variables_test::__spin_wasi_http::<impl vault_variables_test::__spin_wasi_http::preamble::exports::wasi::http::incoming_handler::Guest for vault_variables_test::__spin_wasi_http::preamble::Spin>::handle::h862241b3afe6b6b9
          13: 0xb13d - vault_variables_test.wasm!wasi:http/incoming-handler@0.2.0-rc-2023-10-18#handle
       note: using the `WASMTIME_BACKTRACE_DETAILS=1` environment variable may show more debugging information
    1: wasm trap: wasm `unreachable` instruction executed

Start by rerunning integration_tests::test_vault_config_provider from the all-integration-tests workflow and compare successful and failing runs. Read integration-variables/src/lib.rs at line 8 and trace the Vault configuration path implicated by the failure. Done means the test passes reliably without the observed 500 response or Vault provider error.