[go_router_builder] Report duplicate route paths at build time (#12399)

Adds build-time detection of routes that resolve to the same URL pattern. `go_router` matches the first route that fits, so a duplicated path silently makes the later route's page unreachable, and `LaterRoute().go(context)` navigates you to `EarlierRoute`'s page instead. Today nothing surfaces that until you run into the issue at runtime.

Reported as a warning by default, and  new `duplicate_route_paths` builder option accepts `warning` (the default), `error`, and `ignore`, so teams whose duplicates are mistakes can fail the build instead.

**Before**, `go_router_builder`'s own example app built clean despite two duplicate registrations:

```
$ dart run build_runner build
[INFO] Succeeded after 14s with 36 outputs
```

**After**:

```
$ dart run build_runner build
W go_router_builder on lib/all_types.dart:
  Duplicate route path detected: DoubleRoute is declared more than once at "double-route/:requiredDoubleField".
```

That duplicate was real. `DoubleRoute` had been registered twice since flutter/packages#2395 (2022) and `DoubleExtensionRoute` since flutter/packages#9458 (2025). Both were unreachable, and this PR removes them. The only change to the generated output is the dead route entries.

What competes for a URL namespace:

- Sibling routes, with path parameters normalized, so `product/:id` and `product/:productId` are one pattern.
- Routes inside shell routes and `StatefulShellRoute` branches, since those own no path of their own and their children compete with the routes around them.
- Relative routes, which own their path like any other route, so `/home/details/edit` and `/home/edit` stay distinct.
- Routes from separate annotations, since the generated `$appRoutes` collects them into one list. Scope is the library, `part` files included.

**Testing:** five new golden inputs in `test_inputs/` cover sibling duplicates, parameter-name-only differences, `StatefulShellRoute` branches, relative routes, cross-annotation duplicates, and the sound same-class grouping case. The golden harness gained two optional companion files per input, documented in the README: `<name>.dart.options` supplies JSON builder options so each severity is exercised through the real option-parsing path, and `<name>.dart.warnings` asserts logged warnings, with an empty file asserting silence.

Fixes flutter/flutter#190741

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