ci: make release version bump idempotent

[mogita]

Why the auto-release flow is stuck

The release workflow has failed on the last 3 PR merges (#39, #40, #42), all with the same fatal error at scripts/release/bump_version.php:152:

Uncaught ReleaseScriptException: Could not update version in composer.json

Two problems stacked together:

1. State drift (trigger). PR feat: [CHA-2956] connection pooling: getstream-php #39 included a manual version bump inside the feature branch (composer.json v7.2.0 -> v7.3.0). So the declared version (7.3.0) is now ahead of the latest git tag (v7.2.0), and no v7.3.0 tag/release exists. The script computes the next version from the latest tag, so every feat/fix merge recomputes v7.2.0 + minor = 7.3.0, which is exactly what composer.json already says.

2. Code brittleness (why drift = permanent deadlock). updateComposerVersion() treated a no-op regex replace ($updated === $raw) as fatal. When the file already holds the target version the replace changes nothing, so the script aborts before tagging/publishing. The sibling updateConstantVersion() never had this flaw.

The fix

Count actual replacements instead of comparing strings: throw only when the version field is genuinely absent (count === 0), and treat an already-correct value as success (idempotent). Mirrors updateConstantVersion().

Verification (local, PHP 8.5)

• Reproduced the exact fatal error against the current repo state before the change.
• After the change: a simulated feat merge exits 0 with correct outputs and leaves composer.json/Constant.php byte-identical when already at target.
• Missing version field still throws (count === 0).

Recovery after merging this

This PR is titled ci: on purpose so merging it does not auto-trigger a release (the stale tag would otherwise drive the version).

After merge, reconcile the tag/composer drift and ship the pending v7.3.0 via the manual path (works because use_current_version=true skips the version bump entirely):

gh workflow run release.yml -f use_current_version=true -f version_bump=minor

That tags v7.3.0 at master, creates the GitHub release, and updates Packagist. From then on the flow is monotonic again (tag v7.3.0 -> next feat 7.4.0, fix 7.3.1).

Process note: avoid hand-editing the version field in composer.json/Constant.php inside feature PRs; the release automation owns those fields.