feat: Add backgroundImageFit and backgroundImageRepeat to ChatBackgroundConfiguration

[Copilot]

Description

ChatBackgroundConfiguration.backgroundImage was hardcoded to BoxFit.fill with no repeat support, making it impossible to tile pattern images (e.g. Telegram-style repeating symbol backgrounds) or use other fit modes.

Adds two optional parameters to ChatBackgroundConfiguration:

• backgroundImageFit (BoxFit?) — how the image is inscribed into the container. Defaults to BoxFit.fill (no behavior change).
• backgroundImageRepeat (ImageRepeat?) — how the image tiles. Defaults to ImageRepeat.noRepeat (no behavior change).

ChatBackgroundConfiguration(
  backgroundImage: 'assets/images/pattern.png',
  backgroundImageFit: BoxFit.none,
  backgroundImageRepeat: ImageRepeat.repeat,
)

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

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