fix(web-surfer): sanitize page metadata in prompts

[pragnyanramtha]

Summary

Fixes #7457.

• sanitize page titles and URLs before interpolating them into Web Surfer prompts
• replace markdown-link formatting around page metadata with explicit <page_title> / <page_url> delimiters
• add focused prompt tests covering control-character flattening, delimiter escaping, truncation, and template formatting

Testing

uv run --project python pytest -q python/packages/autogen-ext/tests/test_websurfer_prompts.py
uv run --project python pytest -q python/packages/autogen-ext/tests/test_websurfer_agent.py::test_run_websurfer_declarative
uv run --project python ruff check python/packages/autogen-ext/src/autogen_ext/agents/web_surfer/_prompts.py python/packages/autogen-ext/src/autogen_ext/agents/web_surfer/_multimodal_web_surfer.py python/packages/autogen-ext/tests/test_websurfer_prompts.py
uv run --project python ruff format --check python/packages/autogen-ext/src/autogen_ext/agents/web_surfer/_prompts.py python/packages/autogen-ext/src/autogen_ext/agents/web_surfer/_multimodal_web_surfer.py python/packages/autogen-ext/tests/test_websurfer_prompts.py
uv run --project python pyright python/packages/autogen-ext/src/autogen_ext/agents/web_surfer/_prompts.py python/packages/autogen-ext/src/autogen_ext/agents/web_surfer/_multimodal_web_surfer.py python/packages/autogen-ext/tests/test_websurfer_prompts.py
git diff --check

Note: uv run --project python pytest -q python/packages/autogen-ext/tests/test_websurfer_agent.py has one local failure in test_run_websurfer: agent._page is None after lazy init. The failure occurs before the changed prompt construction paths and appears local Playwright/browser initialization-related; test_run_websurfer_declarative passes.

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