[ARCHITECTURE] Generalise rejection policy configuration

[jeipollack]

Context

An architectural issue was identified during development of the quality
control pipeline orchestration involving the instantiation of rejection
policies from the rejection policy registry.

The current design supports only one policy type: threshold. The
RejectionPolicyConfig dataclass therefore defines threshold as a
configuration attribute. This limits the configuration model as additional
policy types (e.g. quantile-based policies) may require different parameters.

Generalise rejection policy configuration to support registry-selected policy
implementations and policy-specific parameters.

Scope

Tasks are:

• Add a policy identifier to RejectionPolicyConfig.
• Replace the hard-coded threshold field with a generic params mapping.
• Update parse_rejection_policy_config().
• Update the YAML configuration examples and fixtures.
• Update rejection-policy tests.
• Update rejection policy instantiation to obtain the implementation from the
  configured policy identifier (deferred to the pipeline orchestration PR 240 architecture implement quality control pipeline orchestration #243)