Align CONTRIBUTING.md with actual make test-all rebuild behavior (#4346)

Nightly docs reconciliation found a mismatch: `CONTRIBUTING.md`
described `make test-all` as conditionally building the binary, while
the Makefile always rebuilds via `test-all: build` (`build` is phony).
This PR updates the documentation text to match current behavior.

- **Documentation correction**
- Updated the “Run All Tests” description in `CONTRIBUTING.md` to state
that `make test-all` **always rebuilds the binary first**.
  - No Makefile logic or test behavior was changed.

- **Behavior now described accurately**
- `make test-integration` remains conditionally built if binary is
missing.
- `make test-all` remains unconditional rebuild, now reflected correctly
in docs.

```makefile
test-all: build
	@echo "Running all tests..."
	@go mod tidy
	@go test -v ./...
```

> [!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-build2713270827/b509/launcher.test
/tmp/go-build2713270827/b509/launcher.test
-test.testlogfile=/tmp/go-build2713270827/b509/testlog.txt
-test.paniconexit0 -test.timeout=10m0s -test.v=true g_.a -I x_amd64/vet
--gdwarf-5 metadata -o x_amd64/vet -W SVusT-1Bf 1546016/b287/
x_amd64/vet . --gdwarf2 --64 x_amd64/vet` (dns block)
> - Triggering command: `/tmp/go-build3345081388/b513/launcher.test
/tmp/go-build3345081388/b513/launcher.test
-test.testlogfile=/tmp/go-build3345081388/b513/testlog.txt
-test.paniconexit0 -test.timeout=10m0s 2646��
/tmp/go-build2352646787/b046/vet.cfg .cfg 64/pkg/tool/linux_amd64/vet
go1.25.9 -c=4 -nolocalimports U-uEwzGxi3-_ -uns��
/tmp/go-build2352646787/b135/vet.cfg
/opt/hostedtoolcache/go/1.25.9/x64/src/encoding/-ifaceassert
878737dd1094c2f3-d
64/src/runtime/c/tmp/go-build3345081388/b501/difc.test --local
ache/go/1.25.9/x-test.paniconexit0 64/pkg/tool/linu-test.timeout=10m0s`
(dns block)
> - `invalid-host-that-does-not-exist-12345.com`
> - Triggering command: `/tmp/go-build2713270827/b491/config.test
/tmp/go-build2713270827/b491/config.test
-test.testlogfile=/tmp/go-build2713270827/b491/testlog.txt
-test.paniconexit0 -test.timeout=10m0s -test.v=true /sig.s
elemetry.io/otel-ifaceassert x_amd64/vet --gdwarf-5 credentials
lcache/go/1.25.9-bool x_amd64/vet -p g_.a -trimpath x_amd64/vet -I
telabs/wazero/in-atomic -I x_amd64/vet` (dns block)
> - Triggering command: `/tmp/go-build3345081388/b495/config.test
/tmp/go-build3345081388/b495/config.test
-test.testlogfile=/tmp/go-build3345081388/b495/testlog.txt
-test.paniconexit0 -test.timeout=10m0s estl�� 2646787/b295/vet-errorsas
-trimpath 64/pkg/tool/linu-nilfunc -p encoding/gob -lang=go1.25
64/pkg/tool/linu-tests 2646�� /tmp/go-build2352646787/b046/vet.cfg .cfg
64/pkg/tool/linux_amd64/vet go1.25.9 -c=4 -nolocalimports
64/pkg/tool/linuorigin` (dns block)
> - `nonexistent.local`
> - Triggering command: `/tmp/go-build2713270827/b509/launcher.test
/tmp/go-build2713270827/b509/launcher.test
-test.testlogfile=/tmp/go-build2713270827/b509/testlog.txt
-test.paniconexit0 -test.timeout=10m0s -test.v=true g_.a -I x_amd64/vet
--gdwarf-5 metadata -o x_amd64/vet -W SVusT-1Bf 1546016/b287/
x_amd64/vet . --gdwarf2 --64 x_amd64/vet` (dns block)
> - Triggering command: `/tmp/go-build3345081388/b513/launcher.test
/tmp/go-build3345081388/b513/launcher.test
-test.testlogfile=/tmp/go-build3345081388/b513/testlog.txt
-test.paniconexit0 -test.timeout=10m0s 2646��
/tmp/go-build2352646787/b046/vet.cfg .cfg 64/pkg/tool/linux_amd64/vet
go1.25.9 -c=4 -nolocalimports U-uEwzGxi3-_ -uns��
/tmp/go-build2352646787/b135/vet.cfg
/opt/hostedtoolcache/go/1.25.9/x64/src/encoding/-ifaceassert
878737dd1094c2f3-d
64/src/runtime/c/tmp/go-build3345081388/b501/difc.test --local
ache/go/1.25.9/x-test.paniconexit0 64/pkg/tool/linu-test.timeout=10m0s`
(dns block)
> - `slow.example.com`
> - Triggering command: `/tmp/go-build2713270827/b509/launcher.test
/tmp/go-build2713270827/b509/launcher.test
-test.testlogfile=/tmp/go-build2713270827/b509/testlog.txt
-test.paniconexit0 -test.timeout=10m0s -test.v=true g_.a -I x_amd64/vet
--gdwarf-5 metadata -o x_amd64/vet -W SVusT-1Bf 1546016/b287/
x_amd64/vet . --gdwarf2 --64 x_amd64/vet` (dns block)
> - Triggering command: `/tmp/go-build3345081388/b513/launcher.test
/tmp/go-build3345081388/b513/launcher.test
-test.testlogfile=/tmp/go-build3345081388/b513/testlog.txt
-test.paniconexit0 -test.timeout=10m0s 2646��
/tmp/go-build2352646787/b046/vet.cfg .cfg 64/pkg/tool/linux_amd64/vet
go1.25.9 -c=4 -nolocalimports U-uEwzGxi3-_ -uns��
/tmp/go-build2352646787/b135/vet.cfg
/opt/hostedtoolcache/go/1.25.9/x64/src/encoding/-ifaceassert
878737dd1094c2f3-d
64/src/runtime/c/tmp/go-build3345081388/b501/difc.test --local
ache/go/1.25.9/x-test.paniconexit0 64/pkg/tool/linu-test.timeout=10m0s`
(dns block)
> - `this-host-does-not-exist-12345.com`
> - Triggering command: `/tmp/go-build2713270827/b518/mcp.test
/tmp/go-build2713270827/b518/mcp.test
-test.testlogfile=/tmp/go-build2713270827/b518/testlog.txt
-test.paniconexit0 -test.timeout=10m0s -test.v=true .cfg
olang.org/grpc@v-ifaceassert x_amd64/vet . 1546016/b245/ --64
x_amd64/vet 1546�� .cfg olang.org/grpc@v1.80.0/health/gr-ifaceassert
x_amd64/vet -pthread g/grpc/metadata -fmessage-length--version
x_amd64/vet` (dns block)
> - Triggering command: `/tmp/go-build3345081388/b522/mcp.test
/tmp/go-build3345081388/b522/mcp.test
-test.testlogfile=/tmp/go-build3345081388/b522/testlog.txt
-test.paniconexit0 -test.timeout=10m0s` (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

CONTRIBUTING.md

@@ -87,7 +87,7 @@ make test-integration  # Automatically builds binary if needed
 ```
 
 #### Run All Tests
-Run both unit and integration tests (automatically builds binary if needed):
+Run both unit and integration tests (always rebuilds the binary first):
 ```bash
 make test-all
 ```