fix(rust-guard): remove redundant match arm in author_association_floor_from_str (#3654)

Two match arms in `author_association_floor_from_str` both returned
`vec![]`, making `"FIRST_TIMER" | "NONE"` dead code — fully subsumed by
the wildcard below it.

## Changes

- **`guards/github-guard/rust-guard/src/labels/helpers.rs`**: Drop the
`"FIRST_TIMER" | "NONE"` arm; fold into the wildcard with a clarifying
comment.

```rust
// Before
"FIRST_TIMER" | "NONE" => vec![],
_ => vec![],

// After
_ => vec![], // FIRST_TIMER, NONE, or any unrecognised value
```

No behaviour change. The `lib.rs` duplicate integrity-branch issue noted
in the report was already resolved in the codebase.

> [!WARNING]
>
> <details>
> <summary>Firewall rules blocked me from connecting to one or more
addresses (expand for details)</summary>
>
> #### I tried to connect to the following addresses, but was blocked by
firewall rules:
>
> - `example.com`
> - Triggering command: `/tmp/go-build4291131586/b514/launcher.test
/tmp/go-build4291131586/b514/launcher.test
-test.testlogfile=/tmp/go-build4291131586/b514/testlog.txt
-test.paniconexit0 -test.timeout=10m0s E=3 .cfg
/tmp/go-build406-ifaceassert x_amd64/vet . --gdwarf2 --64 x_amd64/vet
.cfg�� /yaml.v3@v3.0.1/apic.go /yaml.v3@v3.0.1/decode.go x_amd64/vet
ute.go mentation.go -o x_amd64/vet` (dns block)
> - `invalid-host-that-does-not-exist-12345.com`
> - Triggering command: `/tmp/go-build4291131586/b496/config.test
/tmp/go-build4291131586/b496/config.test
-test.testlogfile=/tmp/go-build4291131586/b496/testlog.txt
-test.paniconexit0 -test.timeout=10m0s
/tmp/go-build4291131586/b338/vet.cfg oding@v0.5.4/iso8601/parse.go
oding@v0.5.4/iso8601/valid.go x_amd64/vet --gdwarf-5 nal/protolazy -o
x_amd64/vet 4459�� g_.a ache/go/1.25.8/x64/src/log/log.go x_amd64/vet
--gdwarf-5 4459165/b239/ -o x_amd64/vet` (dns block)
> - `nonexistent.local`
> - Triggering command: `/tmp/go-build4291131586/b514/launcher.test
/tmp/go-build4291131586/b514/launcher.test
-test.testlogfile=/tmp/go-build4291131586/b514/testlog.txt
-test.paniconexit0 -test.timeout=10m0s E=3 .cfg
/tmp/go-build406-ifaceassert x_amd64/vet . --gdwarf2 --64 x_amd64/vet
.cfg�� /yaml.v3@v3.0.1/apic.go /yaml.v3@v3.0.1/decode.go x_amd64/vet
ute.go mentation.go -o x_amd64/vet` (dns block)
> - `slow.example.com`
> - Triggering command: `/tmp/go-build4291131586/b514/launcher.test
/tmp/go-build4291131586/b514/launcher.test
-test.testlogfile=/tmp/go-build4291131586/b514/testlog.txt
-test.paniconexit0 -test.timeout=10m0s E=3 .cfg
/tmp/go-build406-ifaceassert x_amd64/vet . --gdwarf2 --64 x_amd64/vet
.cfg�� /yaml.v3@v3.0.1/apic.go /yaml.v3@v3.0.1/decode.go x_amd64/vet
ute.go mentation.go -o x_amd64/vet` (dns block)
> - `this-host-does-not-exist-12345.com`
> - Triggering command: `/tmp/go-build4291131586/b523/mcp.test
/tmp/go-build4291131586/b523/mcp.test
-test.testlogfile=/tmp/go-build4291131586/b523/testlog.txt
-test.paniconexit0 -test.timeout=10m0s .cfg�� 4459165/b389/_pk-p -I
x_amd64/vet --gdwarf-5 g/grpc/internal//usr/bin/runc -o x_amd64/vet
.cfg�� lhk-/9CAufbOhYsrgo1.25.8 /tmp/go-build406-c=4 x_amd64/vet .
_value&#34;].&#34;\n&#34;; --64 x_amd64/vet` (dns block)
>
> If you need me to access, download, or install something from one of
these locations, you can either:
>
> - Configure [Actions setup
steps](https://gh.io/copilot/actions-setup-steps) to set up my
environment, which run before the firewall is enabled
> - Add the appropriate URLs or hosts to the custom allowlist in this
repository's [Copilot coding agent
settings](https://github.com/github/gh-aw-mcpg/settings/copilot/coding_agent)
(admins only)
>
> </details>

Author: lpcox

guards/github-guard/rust-guard/src/labels/helpers.rs

@@ -917,8 +917,7 @@ pub fn author_association_floor_from_str(
     match normalized.as_str() {
         "OWNER" | "MEMBER" | "COLLABORATOR" => writer_integrity(scope, ctx),
         "CONTRIBUTOR" | "FIRST_TIME_CONTRIBUTOR" => reader_integrity(scope, ctx),
-        "FIRST_TIMER" | "NONE" => vec![],
-        _ => vec![],
+        _ => vec![], // FIRST_TIMER, NONE, or any unrecognised value
     }
 }