feat(plugins): validate environment variable collisions across runtime plugins

Currently, when multiple runtime plugins (e.g., `Torch`, `JobSet`, `Volcano`, `TrainJobStatus`) execute during the runtime reconciliation pipeline, environment variables are injected into container specifications via `apply.UpsertEnvVars`.
However, there is no validation step to detect if two plugins attempt to inject conflicting or colliding environment variable names with different values into the same container.

Reference TODO comment in `torch.go`:
TODO (andreyvelich): We should validate that envs from different plugins don't conflict with each other.
Ref: https://github.com/kubeflow/trainer/pull/2308#discussion_r1823229940

Proposed Solution -
Introduce a validation check in pkg/runtime/framework/core during plugin pipeline execution to track injected environment variable keys per container.
Emit an error or validation warning when an environment variable key is overridden by a subsequent plugin with a conflicting value.
Add unit test coverage in pkg/runtime/framework/core/framework_test.go.
Target Files to Change
pkg/runtime/framework/plugins/torch/torch.go (line 171)
pkg/runtime/framework/core/framework.go
pkg/runtime/framework/core/framework_test.go

Assignment Request -
I would like to work on this issue. Please assign it to me!

Start in pkg/runtime/framework/core/framework.go by tracing plugin pipeline execution and read the existing tests in pkg/runtime/framework/core/framework_test.go. Review the TODO at pkg/runtime/framework/plugins/torch/torch.go:171 and the existing apply.UpsertEnvVars behavior. Done means conflicting environment variable injections are detected during execution and covered by unit tests.

Written by the indexing model from the issue text.