⚡ perf: preallocate slice capacity in anthropic messages loops

[matdev83]

💡 What: The optimization preallocates capacity for slice out variables in buildAnthropicMessages, assistantMessageParam, and userPartsToBlocks in internal/plugins/backends/protocols/anthropicmessages/invoke.go.
🎯 Why: To prevent multiple slice reallocations inside loops where the final length of the slice can be determined upfront via len(input_slice).
📊 Measured Improvement:
Before the change: userPartsToBlocks ran in ~31500 ns/op, producing 108 allocations, consuming ~55856 B/op memory on a 100-element input test slice.
After the change: userPartsToBlocks runs in ~18000 ns/op, producing 101 allocations, consuming ~29184 B/op memory on a 100-element input test slice.

This represents a ~42% reduction in operation time and a nearly ~48% reduction in memory allocation for that hot path snippet.

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PR created automatically by Jules for task 9246238034975807275 started by @matdev83

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📥 Commits

Reviewing files that changed from the base of the PR and between 02325fe and 054cb0e.

📒 Files selected for processing (2)

• internal/plugins/backends/protocols/anthropicmessages/invoke.go
• internal/plugins/backends/protocols/anthropicmessages/invoke_bench_test.go

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