Hide "Run Python File" button on .pyi stub files (microsoft#26026)

The editor-title "Run Python File" buttons appear on `.pyi` stub files,
which cannot meaningfully be executed, because `.pyi` and `.py` share
`resourceLangId == python`.

### Change

- In `package.json`, tighten the `when` clauses of both
`editor/title/run` contributions (`python.execInTerminal-icon`,
`python.execInDedicatedTerminal`) to exclude `.pyi`:

```jsonc
"when": "resourceLangId == python && resourceExtname != .pyi && !isInDiffEditor && !virtualWorkspace && shellExecutionSupported"
```

### Notes

- Applied as a sensible default rather than a new setting: stub files
are type-hint-only and not runnable, so no user-facing configuration is
warranted.
- Scoped strictly to the two editor-title run buttons; other Python
command contributions (command palette, explorer context, REPL, etc.)
are unchanged.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Eleanor Boyd <26030610+eleanorjboyd@users.noreply.github.com>

Author: Copilot

package.json

@@ -1441,13 +1441,13 @@
                     "command": "python.execInTerminal-icon",
                     "group": "navigation@0",
                     "title": "%python.command.python.execInTerminalIcon.title%",
-                    "when": "resourceLangId == python && !isInDiffEditor && !virtualWorkspace && shellExecutionSupported"
+                    "when": "resourceLangId == python && resourceExtname != .pyi && !isInDiffEditor && !virtualWorkspace && shellExecutionSupported"
                 },
                 {
                     "command": "python.execInDedicatedTerminal",
                     "group": "navigation@0",
                     "title": "%python.command.python.execInDedicatedTerminal.title%",
-                    "when": "resourceLangId == python && !isInDiffEditor && !virtualWorkspace && shellExecutionSupported"
+                    "when": "resourceLangId == python && resourceExtname != .pyi && !isInDiffEditor && !virtualWorkspace && shellExecutionSupported"
                 }
             ],
             "explorer/context": [