fix: make get_html truncation configurable instead of hard-coded

[xlyoung]

Problem

The get_html browser action had a hard-coded 1,000-character truncation limit that returned broken HTML fragments:

truncated_result = result[:1000] + "..." if len(result) > 1000 else result

This caused issues because:

1. 1,000 characters is too small for most real-world HTML content
2. Truncating at a character boundary breaks HTML structure (mid-tag)
3. Users had no way to control or disable the truncation

Fix

• Add optional max_length parameter to GetHtmlAction model
• Only truncate when max_length is explicitly set by the user
• Default is None (no truncation, return full content)
• Truncation message now includes "...[truncated]" indicator for clarity

Before

# Hard-coded truncation at 1000 chars
truncated_result = result[:1000] + "..." if len(result) > 1000 else result

After

# Only truncate if user explicitly sets max_length
if action.max_length is not None and len(result) > action.max_length:
    result = result[:action.max_length] + "...[truncated]"

Testing

Existing tests pass. The browser tests require bedrock_agentcore which is not available in the test environment (pre-existing issue).

Fixes #407