[Nexthop] [fboss2] don't stage a config session from read-only commands

[hillol-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
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Slack thread

Summary

fboss2-dev config history and fboss2-dev config session diff are purely informational, but both create a config session as a side effect.

Both call ConfigSession::getInstance(), which lazily default-constructs the singleton, whose constructor unconditionally runs initializeSession(). With nothing staged, that takes the "starting a new session" branch: ensureDirectoryExists(~/.fboss2), copySystemConfigToSession() (seeding ~/.fboss2/agent.conf from the live /etc/coop/agent.conf), base_ = git HEAD, and saveMetadata() (writing ~/.fboss2/cli_metadata.json).

Consequences:

• Running config history on a clean box silently starts a config session. config session diff/commit afterwards behave as if the user has an edit in flight.
• session diff's own "No config session exists. Make a config change first." message is checked after getInstance() has already created the session — the message is false by the time it is printed.

Fix

Add a ConfigSession::SessionInit { CreateIfAbsent, ReadOnly } mode, threaded through the constructors into initializeSession():

• ReadOnly returns before the "start a new session" branch, so nothing under ~/.fboss2 is written. The git handle, path getters, configDomains() and readStagedContent() all still work, which is everything these two commands actually use.
• CreateIfAbsent remains the default, so every editing command is byte-for-byte unchanged.
• loadConfig() — the entry point of every editing command — now calls initializeSession(CreateIfAbsent) explicitly, so even a ReadOnly-constructed singleton materializes a session the moment someone genuinely reads the agent config.
• Call sites: CmdConfigHistory and CmdConfigSessionDiff pass SessionInit::ReadOnly.

Two deliberate choices worth reviewing:

1. initializeGit() still runs in ReadOnly mode. It only bootstraps the system config repo under /etc/coop (never ~/.fboss2), it is idempotent once the repo exists, and both read commands need a usable repo to report anything at all. Skipping it would make history fail with a raw git error on a fresh device instead of showing the baseline commit. Happy to tighten this further if reviewers prefer.
2. ~/.fboss2 is now ensured in saveConfig() and saveMetadata() rather than only at construction, since the directory is no longer guaranteed to have been created up front. This also removes a pre-existing latent failure in the rollback() path, which calls saveMetadata() unconditionally.

A "delete the files on exit" approach was considered and rejected: it races against a concurrent config session start/edit in another terminal, and does a pointless copy-then-delete on every invocation. Not creating the file at all is strictly better.

Test Plan

New unit tests (all use a new CmdConfigTestBase::setupReadOnlyTestableConfigSession() helper that installs a SessionInit::ReadOnly session, mirroring how the commands construct theirs):

CmdConfigHistoryTest.cpp

• historyDoesNotCreateSessionFiles — asserts ~/.fboss2 does not exist beforehand, runs the command, asserts the output still lists Initial commit, and that agent.conf, cli_metadata.json, bgp_config.json and hasActiveSession() are all still absent/false.
• historyLeavesExistingSessionUntouched — with a session already staged, the command still works and the staged content is byte-identical afterwards.
• defaultSessionStillCreatesSessionFile — regression guard that the default CreateIfAbsent editing path still seeds ~/.fboss2/agent.conf.

CmdConfigSessionDiffTest.cpp

• diffNoSessionDoesNotCreateSessionFiles — the "No config session exists" message is now true when printed; no session files created.
• diffTwoRevisionsDoesNotCreateSessionFiles — the revision-vs-revision mode never consults the session, and no longer stages one.
• diffReadOnlyStillSeesStagedSession — a real staged session is still diffed correctly and left untouched.

Existing CmdConfigHistoryTest/CmdConfigSessionDiffTest/CmdConfigSessionTest/ConfigSessionSystemdTest cases exercise the unchanged CreateIfAbsent path.

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D115030926. (Because this pull request was imported automatically, there will not be any future comments.)