feat(config): rename GALILEO_* env vars to SPLUNK_AO_* (HYBIM-713)

[adityamehra]

Summary

Renames all `GALILEO_` environment variables to `SPLUNK_AO_` as part of the
Splunk Agent Observability rebrand.

Jira tickets: HYBIM-713
Mirror of: rungalileo/galileo-python#597

This is the authoritative version of this change for the `splunk-ao-python` repo.
The `rungalileo/galileo-python` PR (#597) is kept for upstream awareness.

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Environment variable mapping

Old (`GALILEO_*`)	New (`SPLUNK_AO_*`)
`GALILEO_CONSOLE_URL`	`SPLUNK_AO_CONSOLE_URL`
`GALILEO_API_KEY`	`SPLUNK_AO_API_KEY`
`GALILEO_PROJECT`	`SPLUNK_AO_PROJECT`
`GALILEO_LOG_STREAM`	`SPLUNK_AO_LOG_STREAM`
`GALILEO_LOGGING_DISABLED`	`SPLUNK_AO_LOGGING_DISABLED`
`GALILEO_INGEST_BETA_DISABLED`	`SPLUNK_AO_INGEST_BETA_DISABLED`
`GALILEO_USERNAME`	`SPLUNK_AO_USERNAME`
`GALILEO_PASSWORD`	`SPLUNK_AO_PASSWORD`
`GALILEO_SSO_PROVIDER`	`SPLUNK_AO_SSO_PROVIDER`
`GALILEO_CURRENT_PROJECT_ID`	`SPLUNK_AO_CURRENT_PROJECT_ID`
`GALILEO_TIER`	`SPLUNK_AO_TIER`
`GALILEO_MAX_QUEUE_SIZE`	`SPLUNK_AO_MAX_QUEUE_SIZE`
`GALILEO_NUM_WORKERS`	`SPLUNK_AO_NUM_WORKERS`
`GALILEO_TIMEOUT_SECONDS`	`SPLUNK_AO_TIMEOUT_SECONDS`

Hard cut-over — only `SPLUNK_AO_*` variables are supported. No aliases.

`galileo-core` compatibility bridge

`galileo-core` (upstream private dependency) still reads `GALILEO_` variables.
`SplunkAOConfig.get()` calls `bridge_env_vars()` which mirrors `SPLUNK_AO` → `GALILEO_*`
at runtime until `galileo-core` is updated separately.

Files changed

• `src/galileo/configuration.py` — 14 `ConfigKey.env_var` fields
• `src/galileo/config.py` — auth error messages + `_bridge_env_vars()` bridge
• `src/galileo/utils/env_helpers.py` — direct `os.getenv` calls
• `src/galileo/utils/decorators/telemetry_toggle.py`
• `src/galileo/logger/logger.py`
• `docs/SPLUNK_AO_ENV_RENAME.md` — spec document (new)
• All test files updated to use `SPLUNK_AO_*`

Test plan

• `poetry run pytest` — 2015 passed, 5 skipped
• E2E tests pass (one pre-existing server-side 404, unrelated)

[fercor-cisco]

Approved. I will create tickets for 2 follow-ups:

• Minor — stale bridge after reset() + credential change. The bridge only writes GALILEO_X when absent. Once set, galileo-core's reset() does not clear those env vars, so a get() → reset() → change SPLUNK_AO_* → get() sequence silently re-reads the stale key. Unit tests miss it because the auth guard reads SPLUNK_AO_* directly.
• GALILEO_STAGE_NAME, GALILEO_STAGE_ID, GALILEO_STAGE_VERSION are not bridged and have no SPLUNK_AO_* equivalent anywhere in configuration.py.