feat(bigtable): add NoOpChannelPrimer for session channel pools - #20208

…ols (#20209)

## Summary

Adds a session-pool sibling to `pingAndWarmDirectAccessChecker`. Session
channel pools do not use PingAndWarm — they warm channels via the
`OpenSession` handshake on each newly-opened stream (see #20208 —
`NoOpChannelPrimer`). Passing a NoOp primer into the classic checker
would leave `isALTSConn` unset, so the ALTS check would always fail for
session pools.

`getClientConfigDirectAccessChecker` runs the same `CheckCompatibility`
flow (dial → probe → ALTS check → success-metric or async
investigation), but issues `GetClientConfiguration` as the probe RPC —
the same verb session pools already talk on the wire.

## Changes

- **`direct_access_checker.go`** (modified): extract two shared helpers
from `pingAndWarmDirectAccessChecker` so both checkers can use them
without duplication.
- `investigateDirectAccessFailure(logger, reportFailure, probeSingle,
originalErr)` — the GCE-environment precondition walk, now taking a
`probeSingle` callback so each checker plugs in its own RPC-verb probe.
- `newAltsProbeChannel(ctx, targetEndpoint)` — the ALTS + oauth +
authority-override dial used by the single-endpoint investigation probe.
Behavior unchanged.
- `pingAndWarmDirectAccessChecker.investigateFailure` /
`.probeSingleEndpoint` become thin wrappers over the shared helpers.
Existing behavior preserved.
- **`direct_access_checker_getclientconfig.go`** (new):
`getClientConfigDirectAccessChecker` struct, constructor,
`CheckCompatibility`, `probeGetClientConfig` (the compatibility probe),
`probeSingleEndpoint` (the single-endpoint investigation probe), and
`recordProbePeer` — the ALTS + IP-protocol side-effect helper that
mirrors what `BigtableConn.Prime` does for PingAndWarm.
- **`direct_access_checker_getclientconfig_test.go`** (new): 7 tests
covering interface satisfaction, dialer identity, dial-failure
short-circuit, and the four IP/ALTS observation branches of
`recordProbePeer`.

## Test plan

- [x] \`go test ./bigtable/internal/transport/ -run
'GetClientConfigDirectAccess|RecordProbePeer' -count=1 -short\` → 7/7
pass.
- [x] \`go test ./bigtable/internal/transport/ -count=1 -short\` (full
transport suite, ~200 tests, 42s) → all pass, verifying the pingAndWarm
refactor is behavior-preserving.
- [x] \`go build\` / \`go vet\` / \`golint\` all clean.
- [ ] CI green.

## Follow-up

A subsequent PR will add a session-pool factory that wires this checker
alongside `NoOpChannelPrimer` (from #20208) and the
`ClientConfigurationManager` polling loop.

🤖 I have created a release *beep* *boop*
---


##
[1.52.0](bigtable/v1.51.0...bigtable/v1.52.0)
(2026-08-03)


### Features

* **bigtable:** Add AFE picker (Simple / LeastInFlight / LeastLatency)
([#20204](#20204))
([bcbf714](bcbf714))
* **bigtable:** Add ClientConfig.DisableSession to opt out of session
backend
([#20297](#20297))
([7ee5e44](7ee5e44))
* **bigtable:** Add getClientConfigDirectAccessChecker for session pools
([#20209](#20209))
([3b8d30a](3b8d30a))
* **bigtable:** Add NoOpChannelPrimer for session channel pools
([#20208](#20208))
([d055a8a](d055a8a))
* **bigtable:** Add per-AFE sessionList for the two-tier session pool
([#20224](#20224))
([dbf0c3f](dbf0c3f))
* **bigtable:** Add protoRowToRow conversion helper for TableShim
([#20257](#20257))
([1297143](1297143))
* **bigtable:** Add Session debug surface (observability fields +
methods)
([#20211](#20211))
([d8d3e16](d8d3e16))
* **bigtable:** Add Session lifecycle (Start, Close, ForceClose,
readLoop, heartBeatLoop)
([#20215](#20215))
([b9e53c6](b9e53c6))
* **bigtable:** Add Session struct + state machine
([#20117](#20117))
([09acbb3](09acbb3))
* **bigtable:** Add session.Config.EnableDebug to gate sessionz debug
state
([#20247](#20247))
([ce74c31](ce74c31))
* **bigtable:** Add SessionClient + SessionTable + lazyPool
([#20228](#20228))
([ab2c96c](ab2c96c))
* **bigtable:** Add SessionPoolImpl (two-tier pool + scaling + debug)
([#20225](#20225))
([683eda8](683eda8))
* **bigtable:** Rename session pool display to
<resource-id>-<PERM>
([#20248](#20248))
([35e146e](35e146e))
* **bigtable:** Route Client.Open()-returned *Table through the Diverter
([#20273](#20273))
([2b81c7d](2b81c7d))
* **bigtable:** State-based classification for abnormal session close
([#20243](#20243))
([f2905b7](f2905b7))
* **bigtable:** TableShim fallback to classic on session UNIMPLEMENTED
([#20269](#20269))
([36540af](36540af))
* **bigtable:** TTL-on-idle cache for per-resource session.TableAPI
([#20263](#20263))
([00b2a49](00b2a49))
* **bigtable:** Wire Diverter on Client and route Open* via TableShim
([#20256](#20256))
([b32fbd7](b32fbd7))


### Bug Fixes

* **bigtable:** AFE picker latency signal — subtract poolWait and
compute TransportLatency = wire − backend at source
([#20281](#20281))
([bb8c4d5](bb8c4d5))
* **bigtable:** Guard NewStream OnFinish against grpc-go double-fire
([#20295](#20295))
([b51da29](b51da29))
* **bigtable:** Real per-resource pool teardown on sessionTable.Close +
cache close-race gate
([#20264](#20264))
([599aea9](599aea9))
* **bigtable:** Session.durations / session.uptime — set explicit
histogram bucket boundaries
([#20276](#20276))
([97eee22](97eee22))
* **bigtable:** SessionTableHandle self-heals across cache eviction
([#20296](#20296))
([0dd98cd](0dd98cd))
* **bigtable:** Translate ctx errors to gRPC status on session vRPC
([#20299](#20299))
([0f3b2a5](0f3b2a5))
* **bigtable:** Treat PingAndWarm NotFound as a successful prime
([#20219](#20219))
([a1557ad](a1557ad))


### Performance Improvements

* **bigtable:** Delete periodic Tick loop; sizing is event-driven
([#20285](#20285))
([2c096bd](2c096bd))
* **bigtable:** Drop pick_lost_race debug tag from CheckoutSession hot
path
([#20280](#20280))
([bd0e400](bd0e400))

---
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