fix(plugin-version): align suggestStrategy base with stableVersion-aware applyStrategy

[KimHyeongRae0]

What's the problem this PR addresses?

Closes #7025.

When a workspace has a prerelease version shadowed by a stableVersion, running yarn version <semver> against a literal target bumps the wrong way. Exact repro from the issue:

```json
{
"name": "test-version",
"version": "1.3.1-alpha",
"stableVersion": "1.2.1"
}
```

```
$ yarn version 1.3.1
➤ YN0000: test-version@workspace:.: Bumped to 1.2.2 ← wrong; should be 1.3.1
```

Root cause is a base-version divergence between two internal paths:

• packages/plugin-version/sources/commands/version.ts picked a release strategy by calling suggestStrategy(workspace.manifest.version, this.strategy). With version="1.3.1-alpha" and arg "1.3.1", semver.inc("1.3.1-alpha", "patch") === "1.3.1" matched, so the suggestion came back as patch.
• packages/plugin-version/sources/versionUtils.ts (inside resolveVersionFiles) deliberately bases the bump on workspace.manifest.raw.stableVersion ?? workspace.manifest.version, per the existing comment "If there's a stableVersion field, then we assume that version contains a prerelease version and that we need to base the version bump relative to the latest stable instead." With stableVersion="1.2.1", semver.inc("1.2.1", "patch") === "1.2.2" — hence the wrong bump.

The two paths disagreed on which version to treat as the base.

How did you fix it?

Mirror the stableVersion-aware base in the suggest step so both paths agree on the baseline:

```ts
// packages/plugin-version/sources/commands/version.ts
const baseVersion = workspace.manifest.raw.stableVersion ?? workspace.manifest.version;
const suggestedStrategy = versionUtils.suggestStrategy(baseVersion, this.strategy);
```

When no pre-defined strategy matches the stable base (as in the #7025 case), the literal semver argument is preserved — applyStrategy already short-circuits on semver.valid(strategy) and returns it verbatim.

For the #7025 scenario this restores the expected flow:

• baseVersion = "1.2.1", no strategy fits → releaseStrategy = "1.3.1" (literal).
• applyStrategy(..., "1.3.1") → "1.3.1". Post-bump applyReleases removes the now-obsolete stableVersion field because semver.prerelease("1.3.1") === null.

A regression test is added under packages/acceptance-tests/pkg-tests-specs/sources/commands/version.test.ts covering the exact repro. All existing tests in that file (20 pre-existing + the new one) pass. Changeset shape follows #6879 (the most recent plugin-version-scoped PR): @yarnpkg/cli: patch + @yarnpkg/plugin-version: patch.

Checklist

• I have read the Contributing Guide.
• I have set the packages that need to be released for my changes to be effective.
• I will check that all automated PR checks pass before the PR gets reviewed.