feat: populate RunSpec.related_to on rerun and in-container flyte.run (#1279)

## Why

The backend added `RunSpec.related_to` (field 15,
`flyteidl2.common.RunIdentifier`): an optional, set-at-creation pointer
to the single parent run this run was derived from (re-run, recover,
clone, or explicit provenance). This PR makes the SDK populate it
automatically at CreateRun time.

## What

Two implicit provenance sources, both resolved by a new
`_Runner._resolve_related_to` helper and stamped in `_apply_overrides`:

1. **Rerun** — every rerun surface (`flyte rerun`, `flyte run
--rerun-from`, `flyte.rerun`, `with_runcontext().rerun()`) converges on
`_Runner.rerun`; the new run points at the run being rerun. The source
id comes from the fetched `action_details.pb2.id.run`, with a fallback
to the `run_name` argument when the fetched id is empty.
2. **In-container `flyte.run`** — when a run is created from inside a
running remote task (`TaskContext.is_in_cluster()`), the new run points
at the invoking run (`flyte.ctx().action`). Local and hybrid executions
are excluded; in a rerun the explicit source wins over the ambient task
context.

### Gating & scope rules

- **Descriptor-gated, silent skip**: the published flyteidl2 (2.0.26)
doesn't ship the field yet. Unlike `recover` — explicitly
user-requested, so its absence raises — implicit provenance silently
skips with a debug log, so reruns keep working on older builds.
- **Same-scope, complete-id only**: per the proto contract `related_to`
is scoped to the new run's org/project/domain, and all four
`RunIdentifier` fields are server-required (buf.validate `min_len=1`).
The resolver stamps only when the source scope (empty fields inherited
from the init config) equals the new run's target scope and the id is
complete — a provenance pointer must never fail run creation.
- **Rerun base-copy path**: the field is unconditionally cleared before
stamping, so a rerun-of-rerun overwrites the inherited grandparent
pointer, and a cross-scope rerun (e.g.
`with_runcontext(project="other").rerun(...)`) clears it rather than
propagating a wrong-scope pointer.

### Tests

- New `tests/flyte/test_related_to.py`: resolver rules (ctx detection,
mode gating, explicit-source precedence, scope mismatch, empty-org,
cfg-scope leniency) — runs on any flyteidl2 build.
- `test_run_runspec_chars.py` / `test_rerun.py`: wire-level tests gated
on `RunSpec.DESCRIPTOR.fields_by_name` — they skip on 2.0.26 and were
verified to pass against the generated protos that ship the field
(stamping, overwrite, clear, name fallback). Two inverse-gated tests pin
the silent no-op on old builds and flag when the gate becomes obsolete.
- `_init_for_testing` gains an `org=` parameter (stamping requires a
non-empty org); existing tests init without org and are unaffected — the
RunSpec snapshot oracle stays byte-for-byte identical.

### Notes for reviewers

- **Backend caveat**: if the server stamps `related_to` itself, a rerun
performed with an *old* SDK fetches the base spec carrying field 15 as
an unknown field, which `CopyFrom` preserves and `ClearField` cannot
touch — the backend should treat client-sent `related_to` at create time
as advisory.
- Follow-ups deliberately out of scope: explicit
`with_runcontext(related_to=...)`, provenance for trigger-created runs.

---------

Signed-off-by: Ketan Umare <kumare3@users.noreply.github.com>
Co-authored-by: Ketan Umare <kumare3@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: kumare3