enhance(sdk): validateMcpb should verify entry_point is a regular file matching server.type

[mgoldsborough]

Background

After PR #94, `validateMcpb` rejects unsafe `entry_point` strings via the schema. But the existence check (`existsSync(join(tempDir, entry_point))`) doesn't verify:

1. The path is a regular file (could be a directory)
2. The file is appropriate for `server.type` (e.g., a `server.type: "node"` bundle pointing at `main.py` is logically broken)

Proposed checks

• `statSync(...).isFile()` — `entry_point` must be a regular file
• Type-aware extension hints (warning, not error):
  • `node`: `.js`, `.cjs`, `.mjs`
  • `python`: `.py`, or module path with no extension (e.g., `mcp_echo.server`)
  • `binary`: any (must also be executable per Unix mode bits)
  • `uv`: same as python

The type-aware check is a soft hint; some Python bundles use module-style entry points like `mcp_echo.server` which have no extension.

Open design question

Returning soft warnings means extending `McpbValidationResult` from a binary success/failure to a tri-state (or adding a `warnings: string[]` field on success). Worth discussing before implementation.

Acceptance criteria

• `validateMcpb` rejects bundles whose `entry_point` is a directory
• Tests in `validate.test.ts` cover the directory case
• Type-extension mismatch produces a warning (or design alternative agreed on the issue)