(WIP) feat: add sentry_merge_context

[JoshuaMoelans]

Adding this for downstream SDKs, so they can avoid overwriting pre-set context from our native SDK init.

Popped up as part of https://github.com/getsentry/sentry-xbox/pull/151 where the unreal integration test calls AddDeviceContext which does a plain set_by_key, making us lose device.X context set by the console SDK.

[sentry]

Bug: The return value of sentry__value_merge_objects is ignored, which could lead to silent data loss if the merge operation fails.
_{Severity: LOW}

Suggested Fix

Check the return value of sentry__value_merge_objects. If it indicates an error, consider logging the failure to make the code more robust and aid in debugging potential issues with context merging.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: src/sentry_core.c#L1086-L1087

Potential issue: In `sentry_merge_context` and `sentry_merge_context_n`, the return
value of the internal function `sentry__value_merge_objects` is not checked. This
function can return an error if it fails, for example, if the destination object is
frozen or if a memory allocation fails. While the analysis indicates that such failure
conditions are unlikely in normal production use, ignoring the return value creates a
defensive coding gap. If a merge operation were to fail, it would do so silently,
potentially leading to incomplete context data being processed without any indication of
an error. The reference counting is correct, so no memory leak is introduced, but data
could be lost.

Also affects:

• src/sentry_core.c:1101~1102

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[jpnurmi]

Hmm, shouldn't downstream values take precedence? For example, passing "Xbox" should override "PC", right? 🤔

[JoshuaMoelans]

In general there are definitely values that should take precedence (e.g., on detecting steamOS for example), but these could then still use the normal SetContext. That would lose the 'merge' for other OS context though, so maybe alternatively we could add a flag to our merge called "overwrite_dst" that says whether to keep values of existing keys, or overwrite them 🤔

[jpnurmi]

What if we made the OS/Device context extensible by allowing downstream SDKs to provide the initial device context as a starting point, and then internally merge sentry-native's OS context into that?