fix: Avoid error when Raw button is absent on GitHub file pages

[sentry]

This PR addresses the "Error: Raw button not found" issue (BROWSER-EXTENSION-SJ) that occurred when the browser extension was active on GitHub file pages that do not display a 'Raw' button (e.g., binary files, images, LFS-tracked files).

Root Cause: The extension's logic assumed the presence of a [data-testid="raw-button"] element on all /blob/ URLs. However, GitHub only renders this button for plain-text files. When navigating to other file types, the document.querySelector call would return null, leading to an unhandled error.

Changes Made:

1. src/content/github/file/main.tsx: Added an explicit check for the [data-testid="raw-button"] element within the main() function. If the button is not found, the extension now logs a message and returns early, preventing createCoverageButton() from being called and thus avoiding the error.
2. src/content/github/file/utils/dropdown.tsx: Modified the createDropdown() function to return null instead of throwing an error if the [data-testid="raw-button"] is not found. This makes the function more robust and allows callers to handle its absence gracefully.
3. src/content/github/file/main.tsx: Updated the call sites for createDropdown() to check for a null return value. If createDropdown() returns null, the subsequent logic for creating and attaching the dropdown (flags and components) is skipped, ensuring no further errors occur.

Fixes BROWSER-EXTENSION-SJ

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[codecov]

Bundle Report

Changes will increase total bundle size by 248 bytes (0.04%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
codecov-browser-extension-array-push	660.71kB	248 bytes (0.04%) ⬆️

Affected Assets, Files, and Routes:

view changes for bundle: codecov-browser-extension-array-push

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
background.js	89 bytes	207.75kB	0.04%
githubFile.js	159 bytes	11.62kB	1.39%

Files in githubFile.js:

• ./src/content/github/file/main.tsx → Total Size: 12.11kB

• ./src/content/github/file/utils/dropdown.tsx → Total Size: 1.91kB