Port pydantic-ai v1.103.0: PreparedToolset nullish-prepare warning + version parity update

[Copilot]

This PR advances the tracked upstream parity from pydantic-ai v1.87.0 to v1.103.0 for the parts relevant to the SDK. From this release span, the applicable runtime change was toolset prepare-callback handling when None/undefined is returned.

• Runtime behavior: PreparedToolset nullish prepare handling

  • Updated PreparedToolset.tools() to handle prepare returning null/undefined.
  • New behavior: emit a warning and fall back to the inner tool list, instead of propagating a nullish result.

• Regression coverage

  • Added a focused test that verifies:
    • warning emission on nullish prepare return
    • fallback tool exposure remains intact for the turn

• Parity/docs/version tracking

  • Bumped .github/pydantic-ai-version.txt to 1.103.0.
  • Updated packages/sdk/docs/reference/features.mdx to document the newly ported prepare-callback warning behavior under Toolsets.

async tools(ctx: RunContext<TDeps>): Promise<ToolDefinition<TDeps>[]> {
  const innerTools = await this._inner.tools(ctx);
  const preparedTools = await this._prepare(ctx, innerTools);

  if (preparedTools === null || preparedTools === undefined) {
    console.warn(
      "PreparedToolset.prepare returned null/undefined; falling back to inner tools.",
    );
    return innerTools;
  }

  return preparedTools;
}