Make optional converters opt-in for MarkItDown MCP

[Sylvester Kaczmarek (sylvesterkaczmarek)]

Fixes #2440.

Depend on base MarkItDown instead of requiring every optional converter. markitdown-mcp[all] provides the previous full converter set, and the Dockerfile explicitly installs that extra to preserve its behavior. Document base, selective, and full installations.

Adds dependency-metadata and HTML-tool regressions, with a clean base-install CI job across Python 3.10 through 3.13. The MCP SDK constraint is unchanged; #2363 remains independent.

Validation:

• Both metadata regressions fail before the change.
• Built wheel; all three tests pass with Python 3.10 and 3.12, including base and full-converter environments on 3.12.
• Installed-wheel STDIO initialization, tool discovery, and HTML conversion pass.
• Python 3.14 / Windows ARM64 binary-only dependency resolution succeeds without the optional PDF/Office stack.
• Black and git diff --check pass.

Native Windows ARM64 execution and a Docker image build were not tested locally.

[afourney]

I'm working to merge #2363 right now. Let me deal with that, then circle back here

[Sylvester Kaczmarek (sylvesterkaczmarek)]

afourney #2363 has now landed, so I rebased this onto current main and resolved the MCP 2.x and CI conflicts while preserving this PR’s opt-in converter scope. The branch is mergeable again.