[action] [PR:24561] [tests/console]: expand custom-escape-char param set for reverse-SSH test

[mssonicbld]

Description of PR

Expand CUSTOM_ESCAPE_CHARS in tests/console/test_console_reversessh.py from ["b", "g", "n", "p", "y"] to ["b", "e", "f", "g", "k", "n", "p", "y"] to broaden coverage of the default_escape_char configurability with three additional readline editing actions:

char	Ctrl-<char>	readline action
e	ENQ (0x05)	end-of-line
f	ACK (0x06)	forward-char
k	VT (0x0B)	kill-line

Same selection rule as the existing entries: pure readline editing actions that are not intercepted by the SSH-client PTY line discipline, so picocom on the DUT sees the raw control byte cleanly.

Char selection — additional exclusions documented

The existing comment block already calls out signal/TTY-eaten letters. This PR adds one more exclusion line covering letters that the SSH client's PTY line discipline rewrites or swallows before they ever reach picocom:

char	reason
h	BS — usually rewritten by ICRNL/ECHO on the SSH client side
i	Tab — TTY processes regardless of picocom mode
j	LF — TTY converts to CR/LF
m	CR — same
v	VLNEXT — swallows the next byte, turning Ctrl-V Ctrl-X into a literal insert

Type of change

• Bug fix
• Testbed and Framework (new/improvement)
• Test case(new/improvement)

Back port request

• 202012
• 202205
• 202305
• 202311
• 202405
• 202411

Approach

What is the motivation?

The previous parametrize set (b, g, n, p, y) covered five chars. Three more readline-only chars (e, f, k) are equally safe and add ~3 min of wall time for non-trivially better feature coverage. Test IDs read [b], [e], [f], [g], [k], [n], [p], [y] — easy to grep / -k filter.

How did you do it?

• Append e, f, k to CUSTOM_ESCAPE_CHARS.
• Update the explanatory comment block above the constant: extend the "selected chars" list with the three new actions, and add a new exclusion bullet for h/i/j/m/v so future contributors know why those letters are off-limits.
• No changes to the fixture, test body, or any other test in the file.

How did you verify it?

• flake8 --max-line-length=120 clean.
• Live verification on a physical c0-lo testbed: 8 passed, 0 errors in 7:25, no teardown drift on CONSOLE_SWITCH|console_mgmt.default_escape_char.

Documentation

N/A.

Signed-off-by: Sonic Build Admin sonicbld@microsoft.com

[mssonicbld]

Original PR: sonic-net/sonic-mgmt#24561

[mssonicbld]

/azp run

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