tikvrpc: add default request origin (#1975)

[ti-chi-bot]

This is an automated cherry-pick of #1975

What changed

• Add a process-wide default request origin in tikvrpc.
• Fill kvrpcpb.Context.request_origin from that default when creating or attaching request context.
• Keep the default as unspecified so non-TiDB client-go users do not get TiDB trust by accident.

Dependency stack

• Uses merged kvrpcpb: add request origin to context pingcap/kvproto#1479 for RequestOrigin and Context.request_origin.
• Ref Make TiKV identify TiDB requests and strengthen TS checks for non-TiDB requests pingcap/tidb#68799.
• No temporary kvproto replace remains; the root module, integration test module, and example modules are bumped directly to the merged kvproto pseudo-version.

Tests

• gotestsum --format short-verbose -- ./tikvrpc
• go test ./tikvrpc -run '^TestDefaultRequestOrigin$' -race
• cd integration_tests && go list -mod=readonly ./...
• for mod in $(find examples -name go.mod -print | sort); do (cd "${mod%/go.mod}" && go list -mod=readonly ./...); done
• git diff --check

Summary by CodeRabbit

• New Features

  • Configurable default request origin for RPCs, ensuring consistent request identification and tracking across operations.

• Chores

  • Bumped an indirect dependency used by examples and integration tests to a newer commit across multiple modules.

• Tests

  • Added unit tests to verify default request origin behavior and that explicit origins are preserved across RPC creation and attachment.

[ti-chi-bot]

@ekexium This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[coderabbitai]

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cfzjywxk, zyguan

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• OWNERS [cfzjywxk,zyguan]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-06-08 06:56:02.610960656 +0000 UTC m=+770263.681278046: ☑️ agreed by cfzjywxk.
• 2026-06-08 07:59:40.281439327 +0000 UTC m=+774081.351756727: ☑️ agreed by zyguan.