fix(cloudflare): avoid duplicating user KV namespaces when injecting SESSION

[greatjourney589]

Closes #16590

Changes

• Fixes a regression introduced in fix(cloudflare): preserve existing KV namespace bindings when injecting SESSION #16555 where user-declared KV namespace bindings (e.g. RATE_LIMIT, CACHE) appear twice in the generated dist/server/wrangler.json, causing wrangler to fail with "<binding> assigned to multiple KV Namespace bindings.".
• The Astro Cloudflare config customizer now returns only the auto-injected SESSION binding from kv_namespaces, instead of pre-merging the user's existing bindings into the output.
• @cloudflare/vite-plugin already merges the customizer's output with the user's wrangler config, so echoing the user's bindings caused the duplication.
• Removed the now-unused withSessionKVBinding() helper.
• Added a changeset (@astrojs/cloudflare patch).

Affected versions reported in the issue: @astrojs/cloudflare@13.3.0–13.3.1 + astro@6.2.x. Last working combination was astro@6.1.9 + @astrojs/cloudflare@13.2.1, which matches the pre-#16555 behavior this PR restores.

Testing

• Updated two existing assertions in packages/integrations/cloudflare/test/wrangler.test.ts that locked in the buggy merged-output shape.
• Added a dedicated regression test does not include user-declared KV bindings in the output (regression #16590) covering the multi-binding scenario from the issue (RATE_LIMIT + CACHE declared by the user) and asserting the customizer returns only [{ binding: 'SESSION' }].
• All 25 tests in wrangler.test.ts pass locally.

Docs

No documentation changes needed. The fix restores documented and previously-shipped behavior — the public API and configuration surface (sessionKVBindingName, automatic SESSION binding provisioning) are unchanged.

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