Fix invalid escape sequences in gpMgmt Python sources - #1969

Python 3.12 promoted "invalid escape sequence" from a DeprecationWarning (hidden by default) to a SyntaxWarning that is printed whenever the module is compiled. On distributions shipping Python 3.12 or newer -- Rocky Linux 10, Fedora 40+, Ubuntu 24.10+ -- the management utilities therefore print warnings before doing any work:

/usr/local/cloudberry-db/lib/python/gppylib/util/ssh_utils.py:268:
SyntaxWarning: invalid escape sequence '\ '
/usr/local/cloudberry-db/bin/lib/pexpect/pxssh.py:105:
SyntaxWarning: invalid escape sequence '['
/usr/local/cloudberry-db/bin/lib/pexpect/pxssh.py:109:
SyntaxWarning: invalid escape sequence '$'
/usr/local/cloudberry-db/bin/lib/pexpect/pxssh.py:110:
SyntaxWarning: invalid escape sequence '$'

The four above come from gpsync/gpssh/gpssh-exkeys and are the ones users see; compiling everything under gpMgmt/ turns up 41 such literals in 16 files. Nothing misbehaves today -- CPython leaves an unrecognised escape in the string as-is, which happens to be what a regex or a shell snippet wants -- but the warnings are noise on stderr, and the Python docs say these sequences will become a SyntaxError in a future release.

Fix the literals rather than silencing the warning:

• a literal whose every backslash escape is invalid gets an r prefix
• a literal that mixes valid and invalid escapes has the invalid backslash doubled instead, since r would also change the meaning of the valid ones

Both transformations leave the literal's runtime value byte-for-byte identical, which was verified mechanically: for each touched file the ordered list of str/bytes constants in the AST is unchanged against the parent commit, and compiling the file under -W error::SyntaxWarning is now clean.

Two of the rewrites are worth a reviewer's eye:

• gpstate_utils.py:79 and replication_slots_utils.py:31 are the mixed-escape case, so they read '\%' and '\A' now.
• the '\A' in replication_slots_utils.py looks like a stray keystroke in a run of shell line continuations -- the shell it is handed sees "&& A ./demo_cluster.sh" and tries to run A. That is pre-existing behaviour, so this commit preserves it exactly rather than quietly changing what the behave step does; it wants a separate fix.

gpMgmt/bin/lib/pexpect is a vendored copy of pexpect 3.3; upstream pexpect made these same literals raw long ago.

27 more files under src/ and contrib/ (mostly gporca and try_convert developer scripts) have the same problem and are left for a follow-up.

Fixes #ISSUE_Number

What does this PR do?

Type of Change

• Bug fix (non-breaking change)
• New feature (non-breaking change)
• Breaking change (fix or feature with breaking changes)
• Documentation update

Breaking Changes

Test Plan

• Unit tests added/updated
• Integration tests added/updated
• Passed make installcheck
• Passed make -C src/test installcheck-cbdb-parallel

Impact

Performance:

User-facing changes:

Dependencies:

Checklist

• Followed contribution guide
• Added/updated documentation
• Reviewed code for security implications
• This PR contains AI-assisted code generation
• Requested review from cloudberry committers

Additional Context

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