Don't render broken links to symbols excluded from documentation (#4448)

[vmishenev]

Summary

Fixes #4448.

When a KDoc/signature link references a symbol that is not part of the rendered documentation (suppressed via perPackageOption/suppress, hidden by skipDeprecated, internal, a private constructor, or an undocumented external symbol), the link resolves to a valid DRI during analysis but the target page is never generated. Dokka silently produced a link to a non-existent page and emitted no warning.

Changes

• Don't render broken links. DefaultExternalModuleLinkResolver.resolve() resolved links purely from a module's package-list, and only verified the target page existed when several local modules shared a package name (the Generating external link should depend on the module it refers to #3368 hack). It now always verifies the page exists — dropping the #anchor first so member links (index.html#anchor) aren't falsely rejected. A missing page yields an unresolved span instead of a dead <a href>.
• Warn developers. Both the multi-module assembly (ResolveLinkCommandHandler) and single-module rendering (HtmlRenderer.buildDRILink) now log a warning when a documentation link cannot be resolved to a page. Assembly-phase warnings are log-only (so multi-module builds aren't broken by unavoidable external links); single-module warnings flow through each module's own failOnWarning.

Tests

• New LinkToExcludedSymbolTest (plugin-base): a public symbol linking to an internal one renders an unresolved span (not <a>), logs a warning, and throws under failOnWarning. Passes on K1 and K2.
• Updated ResolveLinkCommandResolutionTest (+ a negative test where the package resolves but the page is absent) and ResolveLinkGfmCommandResolutionTest for the new existence check.
• IoGradleIntegrationTest: re-enabled assertNoHrefToMissingLocalFileOrDirectory — now passes (kotlinx-io's broken <init> link became a span, and the new "Couldn't resolve link" warnings appear for its external Apple/stdlib links).

Note for reviewers

AbstractIntegrationTest.assertNoUnresolvedLinks is left as-is. Its regex data-unresolved-link="\[ expects a leading [, but real DRI.toString() values (e.g. platform.Foundation/NSData///…) have none, so it currently matches nothing. Fixing it would fail on kotlinx-io's legitimate external-only spans (Apple platform.Foundation.*, kotlin.Throwable members), so it needs a separate decision (allowlist external packages, or drop the assertion).

🤖 Generated with Claude Code