fix(go): select the host CLI package in E2E tests

[Pybsama]

Closes #2116.

What this changes

The Go E2E harness now derives exact @github/copilot-<platform>-<arch>
package candidates from ffihost.PrebuildsFolder() instead of globbing every
copilot-* directory and taking the first sorted result.

• macOS and Windows probe only their exact host package;
• Linux probes the detected glibc/musl package first and the sibling libc
  variant second;
• unrelated or foreign-platform copilot-* packages are ignored;
• CLIPath(testing.TB) reports the packages tried and those actually present,
  replacing the duplicated empty-path guards at its call sites; and
• go/test.sh applies the same exact host OS/architecture mapping.

The shared .github/actions/setup-copilot action is intentionally unchanged,
as called out in the Issue.

Tests

Added unit coverage for:

• macOS, Windows, Linux glibc, and Linux musl candidate generation;
• ignoring alphabetically earlier foreign and non-platform packages;
• respecting Linux libc candidate order;
• rejecting a package without an index.js; and
• listing installed packages for failure diagnostics.

Local validation:

• GOTOOLCHAIN=go1.24.0 go test ./internal/e2e/testharness
• go test ./...
• env -u COPILOT_CLI_PATH bash test.sh
• COPILOT_SDK_DEFAULT_CONNECTION=inprocess GOFLAGS=-tags=copilot_inprocess env -u COPILOT_CLI_PATH bash test.sh
• golangci-lint run ./... --timeout=5m (0 issues)
• bash -n go/test.sh
• cross-compiled the testharness tests for Linux, Windows, and macOS on amd64
  and arm64

Both the default and in-process full race-enabled test runs passed.

[Pybsama]

Revalidated this PR against current main (53efb3593e65e5f04474099ffd3e00efa7777482). Upstream has moved 137 commits since the PR's original base, but the affected CLIPath and go/test.sh discovery blocks still use the loose sorted copilot-* glob, and I found no other open PR for #2116.

The existing PR patch applies cleanly on current main; its stable patch ID is unchanged (c65eba0db789bd46b864cadfe3228471100d46f1). GitHub's current synthetic merge commit 4a58782925d685839fbd94735eebfa6969910ac9 has current main and the public PR head as its two parents, and its tree 0ea94a0a517038537a90d8e5741fa77ea9b73bd6 matches the locally validated current-main tree byte-for-byte. Current-main validation:

• The added focused regression is RED on unmodified current main because the exact platform-candidate helpers are absent; with the patch it passes on Go 1.24.13 and for 50 consecutive runs. It also passes for 20 runs on Go 1.26.5.
• With an alphabetically earlier foreign package present, exact go/test.sh discovery probes selected copilot-darwin-arm64, copilot-linux-x64, and copilot-win32-arm64 for the corresponding simulated hosts.
• Using the workflow's Go 1.24 line (latest patch 1.24.13), official Node 22.23.2, and Copilot CLI 1.0.82, both complete race-enabled matrices passed locally: default transport (322 top-level passes; E2E package 276.743s) and in-process transport (318 top-level passes; E2E package 213.131s). Both selected the Darwin ARM64 package.
• go vet ./..., Go formatting, Bash syntax, golangci-lint 2.12.2 (0 issues), Linux/Windows/macOS testharness cross-compilation on amd64 and arm64, default and in-process Go 1.26.5 compile checks, and git diff --check all pass.
• Public Semgrep rules reported 0 findings across 134 rules on the two changed Go files and go/test.sh.

Both full E2E runs rewrite replay snapshots as part of the harness; those generated changes were restored after each run, and the final source worktree is clean.

I also attempted an exact --force-with-lease update from the unchanged public head 33f12d115a1bb67d03a80c250354ca66780c0665. GitHub rejected it before changing the ref because the intervening upstream history modifies workflows and the current OAuth token lacks workflow scope. I did not expand credential scopes. The public head therefore remains unchanged; GitHub currently reports the PR as mergeable, and the patch above was validated on the current-main equivalent tree.

[SteveSandersonMS]

I independently reproduced the platform-selection bug and validated the fix. The change is confined to Go E2E discovery and is safe to merge.

[SteveSandersonMS]

/test