feat(context): get()/getLocal() throw on a missing key instead of returning null

[EdmondDantes]

The problem

get() was an exact duplicate of find(), and getLocal() of findLocal(). Four methods, two behaviours:

find(parentKey)      'P'      get(parentKey)       'P'
findLocal(parentKey) NULL     getLocal(parentKey)  NULL
                              get(missing)         NULL

Both walked the parent Scopes; both answered null on a miss. Half of the API had no reason to exist.

And three sources described three different things:

Source	Claim
The site docs	"Unlike find(), this method throws if the key is not found at any level. Use get() when the value is mandatory." — and @throws Async\ContextException
The stub	"Get a value by key in the current Context" — i.e. no parent lookup
The code	Identical to find(). Returns null. ContextException does not exist.

The site was describing the useful contract — the standard find / get split (cf. PSR-11) — and the code simply never implemented it.

The change

• find() / findLocal() — the optional-value forms. Unchanged: they answer null.
• get() / getLocal() — the mandatory-value forms. A missing key now raises Async\ContextException, the class the docs already referred to.

$context->find('missing');      // null
$context->get('missing');       // Async\ContextException: Context key "missing" not found
$context->get($objectKey);      // Async\ContextException: Context key of type Foo not found

Parent lookup is untouched — get() still reaches inherited Scopes, exactly like find(). Only the miss differs.

Registration order

async_register_context_ce() ran before async_register_exceptions_ce(), so ContextException was handed a NULL parent and tripped Exceptions must implement Throwable. Context registration now follows the exceptions, as Channel's already does.

BC break — deliberate, and two tests updated rather than worked around

• 008-context_get_missing.phpt was literally named "Context: get() returns null for missing keys". It pinned the behaviour we are calling a bug. It now covers the throw, and keeps a find() call so the null half of the contract stays pinned.
• 002-context_inheritance.phpt used getLocal() to show that a local lookup does not reach the parent. That intent is unchanged — under the new contract it is expressed by the throw rather than by a null. findLocal() still pins the null.

Tests

• New 011 — get()/getLocal() throw, find()/findLocal() answer null, present keys still return, ContextException extends AsyncException.
• New 012 — get() reaches parent Scopes; getLocal() does not; object keys are reported by class name.
• context 12/12. Full ext/async/tests: no new failures — the 3 that fail (curl/063, curl/064, io/082) fail identically on a pristine baseline.

The 36 mentions of ContextException in the docs were left alone on purpose while this was pending; they are now correct.

[codecov]

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