Author BEP for native session retry

Sub-issue of #11320 (Epic: Native session retry support for batch sessions).

## Goal

Author BEP-1053 capturing the design for native session retry, so the rest of the implementation work has an accepted reference document to point at.

## Scope

- Reserve BEP number 1053 in `proposals/README.md`.
- Write `proposals/BEP-1053-native-session-retry.md` covering: motivation, current design, proposed `RetryPolicy` schema (modeled after Apache Airflow's parameter surface), failure classification, decision/dispatch flow, data model, API surface, observability, migration/compatibility, implementation plan, decision log, open questions.
- Working draft already exists at `docs/investigation/native-session-retry-plan.md` — promote and refine it into a BEP.

## Out of scope

- Code implementation. That happens in subsequent sub-issues.

## Acceptance

- BEP file lands on `main` with status `Draft`.
- Registry entry added.
- Linked back from Epic #11320.

## Target version

26.5

JIRA Issue: BA-5851