Add otel tracing around handler

[Uttkarsh-Srivastava]

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[Mrsandeep27]

Clean approach to threading tracing through the hook lifecycle. A few notes:

1. tracedResWriteFirstFoldCss / tracedResWriteFirstFoldJS — both are just thin wrappers around res.write(chunk) with the same signature. Consider a single tracedResWrite factory (or pass the span name as an argument) so you don't duplicate the closure for each chunk type. Current setup will also double-count if you ever add a third chunk.

2. traceHandlerHook gates on typeof fn === "function". If a template exports onRouteMatch as undefined (common when a project only opts into some hooks), the fallthrough returns fn unchanged. Good. But you then pass that undefined into safeCall(onRouteMatch, ...) — confirm safeCall handles undefined fn without emitting a misleading "Invalid lifecycle method" line for hooks that are legitimately optional.

3. _handler is now internal-only but it's still async. withObservability must preserve the promise chain for the rejection path (throw (error) => Promise.reject(error) at the bottom). Worth confirming the wrapper awaits or forwards the rejection — otherwise you'll see tracing spans without error status even when the real handler rejected.

4. Unrelated changes — "Google Agent":"Google-Agent" addition in userAgentUtil.js and the indentation touch on line 239 ( const isBot → const isBot) look like drive-by edits. Splitting them into their own commits would make this PR cleaner and easier to revert if tracing needs a rollback later.

5. Nit: Span names like res.write.firstFoldCss use dot notation, but the existing spans in this file use camelCase (getMetaData, renderMarkUp, onRouteMatch). Picking one style keeps trace dashboards scannable.

Direction looks right; mostly cleanup suggestions.