fix: [Bigtable] Existing value SESSION_TYPE_TEST = -1 is changed to SESSION_TYPE_TEST = 9999 in enum SessionType (#9138)

* fix: Existing value `SESSION_TYPE_TEST = -1` is changed to `SESSION_TYPE_TEST = 9999` in enum `SessionType`

PiperOrigin-RevId: 906375259

Source-Link: googleapis/googleapis@f1a8f29

Source-Link: googleapis/googleapis-gen@9fc0d96
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