Fix set() not clearing cached instance on override

[ChiragAgg5k]

Summary

• When set() was called to override an already-resolved dependency, get() still returned the stale cached value from $concrete
• Added unset($this->concrete[$id]) in set() to clear the cache when a dependency is re-registered
• Added test testSetOverridesCachedInstance that resolves a dependency, overrides it via set(), and verifies the new value is returned

Test plan

• All 31 existing tests pass
• New test covers the resolve-then-override scenario

[greptile-apps]

Greptile Summary

This PR fixes a bug where calling set() to re-register an already-resolved dependency would leave a stale entry in $this->concrete, causing subsequent get() calls to return the old cached value. The one-line fix (unset($this->concrete[$id])) is correct and minimal, and is accompanied by a focused regression test.

Key changes:

• src/DI/Container.php: Clears the resolved-instance cache entry for $id whenever set() is called, ensuring the new factory is invoked on the next get().
• tests/ContainerTest.php: Adds testSetOverridesCachedInstance to cover the resolve-then-override scenario that was previously broken.

Notable limitation: The fix only invalidates the cache for the directly overridden dependency. Dependents that were already resolved and cached using the old value of $id (i.e., entries in $this->concrete whose $this->dependencies list includes $id) will continue to return stale results. This is worth documenting or addressing depending on the intended container semantics.

Confidence Score: 4/5

• Safe to merge — the fix is correct and well-tested, with only a minor design gap around cascading cache invalidation.
• The change is a single, targeted line that directly solves the stated bug, all existing tests pass, and a new test covers the exact scenario. The one deduction is for the unaddressed cascading invalidation case (already-cached dependents of an overridden dependency), which could cause subtle bugs in more complex dependency graphs.
• src/DI/Container.php — review the cascading cache-invalidation limitation noted in the inline comment.

Important Files Changed

Filename	Overview
src/DI/Container.php	Adds unset($this->concrete[$id]) in set() to clear the stale cached instance on override. The fix is correct for the direct entry, but doesn't cascade invalidation to already-cached dependents that relied on the old value.
tests/ContainerTest.php	Adds testSetOverridesCachedInstance which directly exercises the resolve-then-override scenario. Well-structured and targets the exact bug fixed.

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