Firestore: Invalid request routing header

Thanks for the amazing library.

#### Environment details

- Programming language: Rust
- OS: MacOS 15.7.2
- Language runtime version: v1.93.0-nightly
- Package version: `firestore`: 0.0.0

#### Issue

I am using the experimental `firestore` library at the moment.

```toml
google-cloud-firestore = { git = "https://github.com/googleapis/google-cloud-rust", rev = "50f65b0c024f080888c62ca33fe19af35dc607e6" }
```

There is an issue with the generated code.

I received this error below:

```bash
Error: Firestore("Batch write failed: the service reports an error with code INVALID_ARGUMENT described as: Invalid request routing header &database
=projects/{project-name}/databases/{database-name}. Please fill in the request header with format x-goog-request-params:project_id={project-name}&database_id={database-name}. Please refer to https://firebase.google.com/docs/firestore/manage-databases#access_a_named_database_with_a_client_library for more details.")
```

After I noticed the issue, I forked the library, `google-cloud-rust`, and tried to see whether the generated code was wrong.

I made some changes to `src/firestore/src/generated/gapic/transport.rs`
```rust
# Before
let x_goog_request_params = [Some(&req)
.map(|m| &m.database)
.map(|s| s.as_str())
.map(|v| format!("database={v}"))]
.into_iter()
.flatten()
.fold(String::new(), |b, p| b + "&" + &p);

# After
let x_goog_request_params = gaxi::routing_parameter::format(&[
// Extract project_id from "projects/{project_id}/databases/{database_id}"
gaxi::routing_parameter::value(
Some(req.database.as_str()),
&[gaxi::routing_parameter::Segment::Literal("projects/")],
&[gaxi::routing_parameter::Segment::SingleWildcard],
&[gaxi::routing_parameter::Segment::Literal("/databases/"), gaxi::routing_parameter::Segment::SingleWildcard],
)
.map(|v| ("project_id", v)),
// Extract database_id from "projects/{project_id}/databases/{database_id}"
gaxi::routing_parameter::value(
Some(req.database.as_str()),
&[gaxi::routing_parameter::Segment::Literal("projects/"), gaxi::routing_parameter::Segment::SingleWildcard, gaxi::routing_parameter::Segment::Literal("/databases/")],
&[gaxi::routing_parameter::Segment::SingleWildcard],
&[],
)
.map(|v| ("database_id", v)),
]);
```

After the manual changes, everything started to work.