feat: Add configurable chatTextFieldHeightFallback to ChatView

[Copilot]

Description

The bottom padding of the message list falls back to a hardcoded 10.0 when the chat text field height cannot be measured from layout, causing the last message to be obscured behind the input field in some cases.

This PR exposes a chatTextFieldHeightFallback parameter on ChatView that developers can use to override this fallback without patching the package source.

Changes:

• ChatView — adds optional chatTextFieldHeightFallback parameter (defaults to existing defaultChatTextFieldHeight = 10.0, no breaking change)
• ChatViewInheritedWidget — carries the fallback value through the widget tree; updateShouldNotify updated accordingly
• extensions.dart (textFieldHeight getter) — prefers the widget-tree-provided fallback over the compile-time constant

ChatView(
  chatController: chatController,
  chatViewState: chatViewState,
  chatTextFieldHeightFallback: 130, // prevents last message hiding behind input
  // ...
)

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 relevant documentation in docs and 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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