bazel/wee8: Run smoke tests in normal //... testing, remove bespoke CI job

[Copilot]

Follow-up to envoyproxy#5050. Smoke tests were tagged manual (excluding them from bazel test //...) and gated behind a standalone wee8-smoke CI job with no RBE, no --config=ci, and leftover debootstrap/qemu-user-static installs — the opposite of the intended outcome. Additionally, missing-sha handling would hard-fail() at repo-setup time, breaking workspace-mode analysis and creating a V8-bump deadlock.

Structural

• Remove tags = ["manual"] from wee8_smoke_test and wee8_smoke_link_aarch64. target_compatible_with already gates them by platform — incompatible targets are silently skipped in wildcard expansion, so manual was redundant and harmful.
• Delete the wee8-smoke CI job and remove it from status needs. The existing test job (bazel test //... //sysroot:sysroots with RBE + --config=ci, across both workspace/bzlmod matrix legs) picks up //v8:wee8_smoke_test automatically once manual is gone.

Workspace-mode fix (wee8_prebuilt.bzl)

Replaced hard fail() on missing sha256 with a non-fatal path that generates:

cc_library(
    name = "wee8",
    target_compatible_with = ["@platforms//:incompatible"],
)

This solves two problems:

1. Workspace-mode analysis: workspace mode has no @abseil-cpp/@icu repos (they're bazel_dep-only); the real BUILD would fail to analyse. The incompatible stub lets //... skip gracefully.
2. V8-bump deadlock: bumping V8_VERSION previously broke every build until shas were populated. Now only targets that explicitly depend on //v8:wee8 fail, with a clear skip/incompatible signal rather than a repo-setup crash. A comment in wee8_prebuilt.bzl documents the bump procedure.

Other fixes

• wee8_smoke_test.cc: replace assert(config != nullptr) (elided under -c opt/NDEBUG) with unconditional null checks; extend to Engine::make for a stronger link check; drop <assert.h>.
• MODULE.bazel: remove redundant repo_name = "abseil-cpp" (already matches the module name).
• bazel/v8/BUILD: fix attribute ordering — srcs before copts per buildifier convention.