fix: Update pre-release usage guidance

[jsonbailey]

Requirements

• I have added test coverage for new or changed functionality — N/A, documentation-only change
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions — N/A, documentation-only change

Related issues

None.

Describe the solution you've provided

Softens the pre-release warning language in all AI package READMEs. Across 4 files:

• packages/sdk/server-ai/README.md
• packages/ai-providers/server-ai-openai/README.md
• packages/ai-providers/server-ai-langchain/README.md
• packages/ai-providers/server-ai-vercel/README.md

Changes in each:

1. Removed the # ⛔️⛔️⛔️⛔️ and # ☝️☝️☝️☝️☝️☝️ emoji header lines
2. Kept the > [!CAUTION] block (preserving the red styling)
3. Replaced "This library is a alpha version and should not be considered ready for production use while this message is visible." with: "This SDK is in pre-release and not subject to backwards compatibility guarantees or semantic versioning. The API may change based on feedback."

The OpenAI provider's separate > [!NOTE] block about the completion API migration is preserved unchanged.

Human review checklist

• Verify that "This SDK" is the preferred phrasing for the provider packages (as opposed to "This package" or "This provider") — the user's template used "This [SDK/feature]"
• Confirm the OpenAI > [!NOTE] block renders correctly after the > [!CAUTION] block without the old emoji separators

Additional context

Companion PR for the Python repo: launchdarkly/python-server-sdk-ai#90

• Link to Devin run: https://app.devin.ai/sessions/eb2400e69a824ad686e0bbd40301971d
• Requested by: @jsonbailey

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