fix: repair ai lookup tests after helm fork api changes

[drey]

Summary

Updates pkg/chart/lookup_ai_test.go to the current vendored Helm fork API so it compiles again. Test-only change: the five TestAI_LocalClientProvider* cases keep their templates, expected values and assertions — only imports, types and the render call site move to their new homes.

Why

The file was written against an older layout of pkg/helm and had gone stale, so the package failed to build:

pkg/chart/lookup_ai_test.go:11:2: no required module provides package
  github.com/werf/nelm/pkg/helm/pkg/chartutil

Three upstream-sync changes broke it:

• pkg/helm/pkg/chartutil is gone — CoalesceValues now lives in pkg/helm/pkg/chart/common/util (chart/common/util/coalesce.go:49).
• pkg/helm/pkg/werf/helmopts was removed outright; this test was the last reference to it anywhere in the repo.
• engine.RenderWithClientProvider (pkg/helm/pkg/engine/engine.go:124) now takes context.Context first and no longer takes a helmopts.HelmOptions argument.

Key changes

pkg/chart/lookup_ai_test.go

• chartutil.CoalesceValues → chartcommonutil.CoalesceValues (pkg/helm/pkg/chart/common/util).
• engine.RenderWithClientProvider(c, vals, provider, helmopts.HelmOptions{}) → helmengine.RenderWithClientProvider(context.Background(), c, vals, provider), in all five tests.
• Chart templates retyped from a chart-package-local File to chartcommon.File, which is what v2chart.Chart.Templates is declared as (pkg/helm/pkg/chart/v2/chart.go:51); chart literals now use v2chart.Chart / v2chart.Metadata.
• Import aliases aligned with pkg/chart/chart_render.go (v2chart, chartcommon, chartcommonutil, helmengine).
• Restored the //go:build ai_tests tag, which had been dropped in the working tree. It matches the two sibling *_ai_test.go files in the package and the AGENTS.md rule for AI-written tests.

No production code is touched, and go mod tidy is a no-op — every import moved within this module.

Verification

task test:go-test paths="./pkg/chart" tags=ai_tests -- -run TestAI_LocalClientProvider -v

All 5 tests and 7 subtests pass. task lint:golangci-lint paths="./pkg/chart/..." tags=ai_tests reports no new findings.

Review focus / risks

• Low risk overall — test-only, and pkg/helm/pkg/engine/engine_test.go:295-395 exercises the same render path with the same expectations, which is the reference for why the untouched assertions (including the moby/templates/<name> output keys) remain valid.
• Confirm the build tag should stay on. With //go:build ai_tests restored, these tests do not run in the default task test:unit, so a future API break here goes unnoticed until someone runs with the tag. That is the existing convention for *_ai_test.go, but it is the one judgement call in this PR.
• Pre-existing lint findings left alone as out of scope: testpackage fires on all three *_ai_test.go files in pkg/chart (they test unexported symbols such as newLocalClientProvider, so chart_test is not an option), and a wsl_v5 whitespace complaint sits in the already-committed pkg/chart/chart_render.go:376.