Make route group prefix optional

[CoryCharlton]

Summary

Makes the prefix argument of the generated route-group registration method optional.

When a handler is decorated with [RouteGroup("Name")], the generator emits:

public static RouteGroupBuilder Map{Assembly}{Name}Endpoints(
    this IEndpointRouteBuilder app,
    [StringSyntax("Route")] string prefix)

Previously prefix was required for named groups — only the default assembly-level group (Map{Assembly}Endpoints) defaulted it to "". This forces callers to pass a prefix even when they'd rather put the full route on each handler, e.g.:

[Handler]
[MapGet("/monitor/assemblies")]
[RouteGroup("Monitor")]
internal static partial class AssembliesEndpoint { /* ... */ }

// today you must write:
app.MapApplicationMonitorEndpoints(string.Empty);
// now you can simply write:
app.MapApplicationMonitorEndpoints();

This change always defaults prefix to "" so the argument is optional for named groups too.

Backward compatibility

Fully backward-compatible — callers that pass a prefix continue to work unchanged; the prefix simply becomes optional.

Changes

• src/Immediate.Apis.Generators/Templates/Routes.sbntxt — always default prefix to "".
• Regenerated the affected RouteGroupTests verified snapshots.
• readme.md — note that the prefix argument is optional.

[Copilot]

Pull request overview

This PR updates the route-group registration method generated by Immediate.Apis so that the prefix parameter is optional for named route groups (not just the default assembly-level group), enabling callers to omit the prefix and specify full routes per-handler instead.

Changes:

• Updated the routes template to always emit prefix with a default value of "".
• Updated RouteGroupTests verified snapshots to reflect the new optional parameter for named groups.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/Immediate.Apis.Tests/GeneratorTests/RouteGroupTests.ValidRouteGroupTest_groupName=Test123Group#RouteGroupBuilder.Test123Group.g.verified.cs	Snapshot updated: named group route builder now has prefix = "".
tests/Immediate.Apis.Tests/GeneratorTests/RouteGroupTests.ValidRouteGroupTest_groupName=Test_Group#RouteGroupBuilder.Test_Group.g.verified.cs	Snapshot updated: named group route builder now has prefix = "".
tests/Immediate.Apis.Tests/GeneratorTests/RouteGroupTests.ValidRouteGroupTest_groupName=123TestGroup#RouteGroupBuilder.123TestGroup.g.verified.cs	Snapshot updated: named group route builder now has prefix = "".
tests/Immediate.Apis.Tests/GeneratorTests/RouteGroupTests.ValidRouteGroupTest_groupName=_TestGroup#RouteGroupBuilder._TestGroup.g.verified.cs	Snapshot updated: named group route builder now has prefix = "".
src/Immediate.Apis.Generators/Templates/Routes.sbntxt	Template updated to always default prefix to "" (making it optional for named groups too).

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