fix(vscode): guard theme colors lookup so a non-standard palette can't crash activation

[bisma-nawaz]

Description

Fixes #12947. Extension activation crashes with a non-standard color theme.

InlineTipManager.createSvgTooltipDecoration guarded only if (this.theme) and then read this.theme.colors["editor.background"]. When the active theme produces a Monaco theme whose colors map is undefined, that read throws TypeError: Cannot read properties of undefined (reading 'editor.background'), which propagates out of the field initializer and kills activation. The two this.theme?.colors["editor.foreground"] reads in createSvgTooltip have the same latent unsafe access.

This makes all three reads null-safe (this.theme?.colors?.[...]), so a theme without the standard colors falls back to the existing default instead of crashing.

Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

Tests

Added extensions/vscode/src/activation/InlineTipManager.vitest.ts, which exercises createSvgTooltipDecoration with a theme whose colors is undefined. Before the change it throws the editor.background TypeError; after, it does not throw (and the colors-present case still works). npx vitest run src/activation/InlineTipManager.vitest.ts: before = 1 failed, after = 2 passed.

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