fix(policy): undo Condition.operator deprecation (#3668)

## Summary

Removes the `deprecated = true` annotation from `Condition.operator` in
`service/policy/objects.proto` and restores its original doc comment,
then regenerates `protocol/go` and the policy docs.

## Why

`Condition.operator` was deprecated in #3580 in favor of the decomposed
`comparison` + `quantifier` (+ `case_insensitive`) fields. The
decomposed evaluator was never wired up on `main` — `EvaluateCondition`
(`service/internal/subjectmappingbuiltin/subject_mapping_builtin.go`)
still switches entirely on `operator`, nothing reads the new fields, and
there is no normalization shim. So the deprecation marker currently only
produces `staticcheck` SA1019 failures in every consumer of
`GetOperator()` (in-repo: `subject_mapping_builtin.go`,
`otdfctl/migrations/namespacedpolicy/canonical.go`; plus downstream
importers of `protocol/go`) with no migration target to move to.

Undoing the deprecation unblocks those consumers. Once the decomposed
evaluation path actually lands, the field can be re-deprecated alongside
a working replacement.

## Scope

- Drops `deprecated = true` only. The field stays optional via
`(buf.validate.field).ignore = IGNORE_IF_ZERO_VALUE`, so request
validation is unchanged and this is **not** a breaking change (`buf
breaking` passes). `required = true` is intentionally not restored.
- Regenerated `protocol/go/policy/objects.pb.go` via remote-pinned
plugins (reproducible across buf CLI versions).
- Surgically updated the `operator` field entry in
`docs/openapi/policy/objects.openapi.yaml` and `docs/grpc/index.html` to
drop the deprecated marker. Full doc regeneration was avoided because
the local doc plugins produce unrelated churn; CI stages docs before its
drift check, so the enforced generated files (`*.pb.go`, connect
wrappers, `enums.gen.go`) are what matter and those match.

## Verification

- `buf lint service` — 0
- `buf breaking service --against origin/main` — 0
- `golangci-lint run --enable-only staticcheck` on
`subject_mapping_builtin` and `otdfctl/.../namespacedpolicy` — 0 issues
- `go build` + `go test ./internal/subjectmappingbuiltin/...` — pass

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Updated the description of the `operator` field in policy condition
docs to reflect its current role.
* Removed outdated deprecation language from API documentation and
schema references.
  * Clarified that `operator` is the evaluation operator for a relation.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Krish Suchak <suchak.krish@gmail.com>

Author: alkalescent