fix: defer browser headers until session startup

[SulimanAbdulrazzaq]

Proposed change

Scrapling currently generates browser headers while importing _config_tools. If browserforge cannot produce headers for the detected browser version, that exception prevents unrelated entry points such as scrapling mcp from starting.

This change defers default user-agent generation until a headless browser session actually needs it and caches one value per browser mode. Explicit user agents and headed sessions keep their existing behavior. Regression tests verify that importing the configuration module does not call browserforge and that on-demand generation preserves the Chromium/Chrome modes and caching behavior.

Validation:

• pytest -o addopts="" tests/fetchers/test_browser_config.py -q (2 passed)
• Ruff check and format check for the three changed files
• mypy and pyright for the two changed source files
• Bandit and Vermin repository checks for the changed files
• real scrapling mcp stdio initialization returned a valid JSON-RPC result
• git diff --check

The exact browserforge exception was reproduced locally with Scrapling 0.4.12, browserforge 1.2.4, and MCP 1.29.0. A macOS runner was not available, so the platform-independent failure path is additionally covered with a deterministic mock.

Type of change:

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Additional information

• This PR fixes or closes an issue: fixes MCP server crashes on macOS at import: ValueError "No headers based on this input from browserforge #394
• This PR is related to an issue: MCP server crashes on macOS at import: ValueError "No headers based on this input from browserforge #394
• Link to documentation pull request: Not applicable

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doc-strings.

[SulimanAbdulrazzaq]

Hi @D4Vinci, when you have a chance, could you please review this PR? Thank you.

[yetval]

Thanks for the thoughtful fix! LGTM I confirmed it resolves the startup issue in #394. One small note: the underlying browser issue can still appear when fetching, and #397 looks like a good complementary fix. This change is still a worthwhile improvement on its own.