This commit integrates the render cache into the prebuilds
reconciliation flow, enabling cache sharing across all prebuild
operations.

Changes:

1. StoreReconciler:
   - Add renderCache field (*dynamicparameters.RenderCache)
   - Initialize cache in NewStoreReconciler()
   - Pass cache to builder in provision() method

2. wsbuilder.Builder:
   - Add renderCache field
   - Add RenderCache(cache) builder method
   - Pass cache to dynamicparameters.Prepare() when available

How it works:
- Single RenderCache instance shared across all prebuilds
- Each workspace build passes the cache through the builder chain
- Cache keyed by (templateVersionID, ownerID, parameterHash)
- All prebuilds use PrebuildsSystemUserID, maximizing cache hits

Expected behavior for a preset with 5 instances:
- First instance: cache miss → full render → cache stored
- Instances 2-5: cache hit → instant return
- Result: ~80% reduction in render operations per cycle

This addresses the resource cost identified in profiling where the
prebuilds path consumed 25% CPU and 50% memory allocations, primarily
from repeated Terraform file parsing.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Adds cache hits, misses, and size metrics to track render cache
performance and enable monitoring of the cache's effectiveness
in reducing expensive terraform parsing operations.

Metrics added:
- coderd_prebuilds_render_cache_hits_total: Counter for cache hits
- coderd_prebuilds_render_cache_misses_total: Counter for cache misses
- coderd_prebuilds_render_cache_size_entries: Gauge for current cache size

The metrics are optional and only created when a Prometheus registerer
is provided to the reconciler.

Implements automatic expiration and cleanup of cache entries:
- 1 hour TTL for cached entries
- Periodic cleanup every 15 minutes to remove expired entries
- Expired entries are treated as cache misses on get()
- Cache properly closed when reconciler stops to clean up goroutine

Uses quartz.Clock instead of time package for testability, allowing
tests to advance the clock without time.Sleep().

This prevents unbounded cache growth while maintaining high hit rates
for frequently rendered template versions.

Updates the cache entry timestamp on every successful cache hit,
implementing LRU-like behavior where frequently accessed entries
stay in the cache longer.

This ensures that actively used template versions remain cached
even if they were initially added long ago, while rarely accessed
entries expire and get cleaned up.

Adds TestRenderCache_TimestampRefreshOnHit to verify that:
- Cache hits refresh the entry timestamp
- Refreshed entries remain valid beyond their original TTL
- Entries eventually expire after no access for the full TTL period

Changes renderCache from a pointer to an interface to eliminate nil checks:

- Created RenderCache interface with get/put methods
- Renamed concrete type to RenderCacheImpl
- Added noopRenderCache for default no-op behavior
- Removed all nil checks in Render() method

This simplifies the code by ensuring renderCache is always present,
either as a real cache with metrics or as a no-op implementation.
The nil checks were inconsistent with other non-lazy-loaded fields
and added unnecessary complexity at each usage site.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

Previously, tests creating StoreReconciler instances would leak cleanup
goroutines because they never called Stop() to close the render cache.
This caused goleak test failures.

Add t.Cleanup() calls to all tests creating reconcilers to ensure the
render cache cleanup goroutines are properly stopped.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

- Add missing context import to metricscollector_test.go
- Add cleanup calls for all remaining NewStoreReconciler instances
  in reconcile_test.go, including those in goroutines

This completes the fix for goroutine leaks in render cache tests.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

The renderCache field in StoreReconciler was using the concrete type
*RenderCacheImpl instead of the RenderCache interface, preventing
proper cleanup of cache goroutines.

Changes:
- Add Close() method to RenderCache interface
- Implement Close() as no-op in noopRenderCache
- Update StoreReconciler.renderCache to use interface type
- Update wsbuilder.Builder.renderCache to use interface type
- Remove unnecessary nil check in reconciler Stop() method

This ensures all render cache cleanup goroutines are properly stopped
when reconcilers are stopped, fixing goleak test failures.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

The reconciler's Stop() method was returning early if the reconciler
was never started (running == false), which meant the render cache
cleanup goroutine was never stopped.

This happened in tests that create reconcilers but never call Run() on
them. When Stop() was called in t.Cleanup(), it would skip closing the
render cache, causing goroutine leaks.

Fix: Move renderCache.Close() to execute before the running check, so
the cleanup goroutine is always stopped regardless of whether the
reconciler was ever started.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>