fix: prevent mutation of input event during temporary state delta stripping by creating a shallow copy

[hanorik]

Fixes an issue where trimTempDeltaState permanently mutated caller-provided event pointers in-place when filtering out temporary state deltas (keys prefixed with temp:).

Changes

• Prevent In-Place Mutation: Updated trimTempDeltaState across inmemory, database, and vertexai session service implementations to create a shallow copy of the event object when temporary state delta keys are stripped.
• Optimize No-Op Case: Added an early return when no temporary keys are removed (len(filteredStateDelta) == len(event.Actions.StateDelta)), avoiding unnecessary struct allocation when the state delta is unchanged.
• Test Coverage: Added unit tests across all three session service packages to verify that input events remain immutable during temporary state delta trimming.

Testing Plan

• Run go test -race ./session/... to verify existing session service tests and new immutability test cases pass.
• Verify no regressions in temporary state delta filtering across inmemory, database, and vertexai session backends.