gpbackman: env-var fallback for --history-db and fail loud when DB is missing

[talmacschen-arch]

Fixes #96

Motivation

When gpbackman backup-info (or any other gpbackman subcommand) is run
without --history-db, the current code drops down to a relative
gpbackup_history.db resolved against the current directory. Two
problems follow:

1. The cluster's real history DB lives under
   $COORDINATOR_DATA_DIRECTORY/gpbackup_history.db (or the legacy
   $MASTER_DATA_DIRECTORY on older installs), which is rarely the
   user's shell cwd. Operators end up typing the long path on every
   invocation despite the relevant env vars already being set by the
   standard cluster environment scripts.

2. SQLite's default open mode silently creates a new empty database
   when the file is missing. The very next query then fails with
   no such table: backups, which is confusing and leaves an empty
   gpbackup_history.db litter in cwd.

Reproducer (current behaviour on main):

$ cd /tmp/empty
$ gpbackman backup-info
[ERROR]:-Unable to read data from history db. Error: no such table: backups
$ ls
gpbackup_history.db   # ← silently created, 0 rows, polluted cwd

Changes

• gpbackman/cmd/wrappers.go — getHistoryDBPath resolves the path
  in order: --history-db → $COORDINATOR_DATA_DIRECTORY →
  $MASTER_DATA_DIRECTORY → bare filename in cwd (preserved for
  backward compatibility).
• gpbackman/gpbckpconfig/utils_db.go — OpenHistoryDB now
  pre-checks the file with os.Stat and opens SQLite via the
  file:<path>?mode=rw URI. Missing files surface a clear error that
  references --history-db and the env vars; no empty file is ever
  created.
• Help text updated for the --history-db flag and every
  command's Long description; gpbackman/COMMANDS.md and
  gpbackman/README.md aligned.
• Tests added/extended in wrappers_test.go (env-var resolution
  order, explicit-wins-over-env) and in utils_db_test.go (friendly
  error + no-file-created regression, happy-path open).

Backward compatibility

Fully compatible. The old "search the current directory" behaviour is
preserved as the last fallback, so any existing scripts that rely on
running gpbackman from a directory containing gpbackup_history.db
continue to work unchanged. Users who currently pass --history-db
explicitly are unaffected.

The only user-visible behaviour change is that running gpbackman
against a non-existent path now errors immediately with a clear message
instead of producing a misleading "no such table: backups" error after
silently creating an empty file. This is the intended fix.

How tested

• go test ./gpbackman/cmd/... ./gpbackman/gpbckpconfig/... ./gpbackman/textmsg/... — all pass.
• make build — five binaries built cleanly.
• Manual smoke test against a real cluster history DB:
  • Empty cwd, no env → friendly error, no file created. ✅
  • COORDINATOR_DATA_DIRECTORY pointing at a non-existent path → friendly error mentioning the resolved path. ✅
  • COORDINATOR_DATA_DIRECTORY pointing at the coordinator data dir → backup list rendered without --history-db. ✅

Notes

• DCO Signed-off-by included on both commits.
• See [Bug] gpbackman: --history-db should fall back to $COORDINATOR_DATA_DIRECTORY, and OpenHistoryDB should not silently create empty files #96 for additional discussion / open questions (env-var name choice, optional GPBACKMAN_HISTORY_DB override).

[MisterRaindrop]

LGTM

[talmacschen-arch]

@woblerr CI s3_plugin_e2e failed in the locale-setup step (dnf 404s
on Rocky 9.7 mirrors — see #issue-link if filed). It's unrelated to
this PR's changes; could you re-run the failed job when convenient?
Thanks!

[talmacschen-arch]

@tuhaihe @woblerr — pushed 2d88689 with the MASTER_DATA_DIRECTORY
cleanup we agreed on (per @woblerr's request and @tuhaihe @MisterRaindrop's
+1). The new workflow runs are sitting in action_required (GitHub's
external-fork CI gate). Could one of you approve them when convenient?

cc @MisterRaindrop for visibility — feel free to re-review once CI clears.

[woblerr]

LGTM, thank you for your efforts.

[MisterRaindrop]

LGTM