bash completion: generate Click 8-compatible scripts with type,value parsing

[anamehra]

Fixes: sonic-net/sonic-buildimage#25882

Use a fixed template instead of Click's source() so generated scripts invoke with bash_complete, redirect stderr, and parse type,value lines to populate COMPREPLY with values only.

What I did

SONiC routers click tab auto completion does not work in master and 202511 images

How I did it

Shell completion (Click 8.x)
https://click.palletsprojects.com/en/8.0.x/shell-completion/
There’s no section titled “bash_complete”. It’s defined by the generic rule for adding a new shell:

• In “Adding Support for a Shell” the doc says the script “must invoke your program with the environment variable _{FOO_BAR}_COMPLETE set to {shell}_complete”.
• For the built‑in shell bash, that becomes bash_complete (i.e. {shell}_complete with shell=bash).
• So:
  bash_source → “output the completion script to source” (run once, user evals/sources it).
  bash_complete → “run in completion mode and output the current completions” (run on each Tab with COMP_WORDS/COMP_CWORD set).

How to verify it

On router do
show
show platform

Previous command output (if the output of a command-line utility has changed)

show platf<tab> does not auto complete
show platform <tab> does not show right options
root@sfd-lc1:/home/admin# show platform <press tab>.
./                         ../                        .bash_history              .bash_logout               .bashrc                    .profile                   .sudo_as_admin_successful 

New command output (if the output of a command-line utility has changed)

root@sfd-lc0:/home/admin# show platform  <tab>
bmc          eth-switch   fan          idprom       leakage      obfl         phy          rdma-config  ssdhealth    syseeprom    versions     
current      fabric       firmware     inventory    npu          pcieinfo     psustatus    security     summary      temperature  voltage      

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).