HtmlScreenshot: honour OFFICECLI_BROWSER as an explicit chrome-family executable

[hesong12]

Files: src/officecli/Core/HtmlScreenshot.cs (FindChrome()), README.md

Summary

FindChrome() only ever looks for a chrome-family binary by PATH name
(google-chrome, chrome, msedge, …) or a fixed set of platform-standard
install paths (/Applications/Google Chrome.app, …). A host that ships its
own Chromium build outside those locations — for example a pinned
"Chrome for Testing" binary staged under an application-support directory —
has no way to point OfficeCLI at it, and screenshot/mermaid rendering fails
with no_screenshot_backend even though a perfectly good browser is sitting
right there.

This PR adds one explicit override, checked first: OFFICECLI_BROWSER. When
set to a path that exists, FindChrome() returns it immediately, before the
PATH and absolute-path fallbacks. When unset (or pointing at a nonexistent
file), behaviour is byte-for-byte unchanged.

private static string? FindChrome()
{
    // Explicit override first: lets hosts that ship their own Chromium
    // (e.g. a Chrome for Testing build outside the standard install
    // paths) point OfficeCLI at it without touching PATH.
    var explicitBrowser = Environment.GetEnvironmentVariable("OFFICECLI_BROWSER");
    if (!string.IsNullOrWhiteSpace(explicitBrowser) && File.Exists(explicitBrowser))
        return explicitBrowser;

    string[] names = [...]; // unchanged from here down
    ...
}

One README line documents it next to the existing description of the
headless-screenshot rendering path.

Why

Product-side rationale for the host that needed this: a desktop app ships
its own pinned Chrome for Testing binary for its browser-automation feature
and wants OfficeCLI's screenshot/mermaid rendering to reuse that exact
binary instead of requiring a second, separately-installed system Chrome.
OFFICECLI_BROWSER is the minimal, additive way to do that — no new
dependency, no behavior change for anyone who doesn't set it.

Validation

Toolchain: .NET 10 SDK installed to a scratch directory (dotnet-install.sh --channel 10.0), never touching the system SDK. dotnet build src/officecli -c Debug succeeds on the pristine clone and on this branch.

Command sequence, run on macOS with a real Chrome for Testing binary at a
non-standard path and the platform-standard Chrome/Chromium/Edge locations
made unreadable for the duration of the test (sandbox-exec denying
file-read* on /Applications/Google Chrome.app, Chromium.app,
Microsoft Edge.app — i.e. "no browser at a standard location", the exact
condition OFFICECLI_BROWSER exists to solve):

officecli create t.docx
officecli add t.docx /body --type paragraph --prop text=hello
officecli close t.docx

# BEFORE (unmodified main, OFFICECLI_BROWSER not yet read by FindChrome()):
OFFICECLI_BROWSER="$CFT_PATH" officecli view t.docx screenshot -o t.png --json
# → {"success":false,"warnings":[{"message":"No headless browser available. ...
#     Last error: playwright: ...", "code":"warning"}]}   (no_screenshot_backend)
# → t.png: No such file or directory

# AFTER (this branch):
OFFICECLI_BROWSER="$CFT_PATH" officecli view t.docx screenshot -o t.png --json
# → {"success":true,"data":".../t.png","message":".../t.png"}

$CFT_PATH = .../chrome-149.0.7827.22/chrome-mac-arm64/Google Chrome for Testing.app/Contents/MacOS/Google Chrome for Testing.

Result PNG: 1600×1200, 8-bit RGB, 9296 bytes — see attached U1-screenshot.png.

Confirmed the override truly changed behavior (not just "a browser happened
to be found"): the identical command against the unmodified main
branch, with the exact same env var set and the exact same browsers hidden,
still fails with no_screenshot_backend — proving FindChrome() does not
read OFFICECLI_BROWSER before this change, and does after.

Compatibility

Additive only. FindChrome()'s existing PATH-name and absolute-path search
is untouched and still runs whenever OFFICECLI_BROWSER is unset.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Prefer the explicit browser before Playwright

For view ... screenshot on a host with a working playwright CLI, this override is never honored: Capture() iterates Backends(), which tries Playwright before TryChrome, and returns immediately when Playwright succeeds. Consequently, operators cannot force the pinned Chrome-for-Testing executable described in the README whenever Playwright is installed; a valid OFFICECLI_BROWSER should make the Chrome backend run before automatic backend discovery while preserving the existing order when the variable is unset.

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