Add support for reading --cli-input-json and --cli-input-yaml from stdin

[SebTardif]

Description

Add support for passing - as the value of --cli-input-json and --cli-input-yaml to read input from standard input. This enables clean pipeline usage:

cat params.json | aws s3api head-object --cli-input-json -
generate_yaml | aws s3api list-objects-v2 --cli-input-yaml -

Fixes #5982
Fixes #7850

Motivation and Context

This has been requested since 2018 (PR #3209, issue #3700, issue #5982, issue #7850). Users need stdin support for:

• Pipeline/CI workflows — generating JSON/YAML dynamically and piping it directly
• Security — avoiding exposure of sensitive values (e.g. secrets) in process tables via command-line arguments (raised by @Nanovox in Enable reading cli-input-json from stdin #5982)

Approach

Following the direction suggested by @stealthycoin in PR #3209:

"I think it makes more sense to make a special case for --cli-input-json where it accepts - as a value that means read stdin. Much like aws s3 cp - s3://mybucket/stream.txt will copy stdin to a bucket."

The implementation adds a - check in CliInputArgument._get_arg_value() before the existing file:// resolution. This:

• Follows the existing s3 cp - convention already established in the CLI
• Is scoped to --cli-input-json / --cli-input-yaml only — avoids interactive mode conflicts raised in PR added support for using stdin and pipes #3209
• Reads stdin exactly once — avoids the double-read problem that breaks file:///dev/stdin workarounds
• Works cross-platform — no /dev/stdin dependency (works on Windows)
• Does NOT add a generic pipe:// or stdin:// scheme — keeps the change minimal and focused

Testing

• 4 new unit tests for JSON stdin: basic read, no-clobber, bad JSON, empty stdin
• 1 new unit test for YAML stdin
• 2 new functional tests for JSON stdin: basic read, CLI arg override
• 1 new functional test for YAML stdin

All existing tests continue to pass. The inherited test structure ensures YAML tests also get the JSON stdin tests via class inheritance.

Changes

File	Change
awscli/customizations/cliinput.py	Add sys import; add - stdin check in _get_arg_value(); update help text for both JSON and YAML arguments
tests/unit/customizations/test_cliinput.py	Add 5 stdin test methods
tests/functional/test_cliinput.py	Add 3 stdin test methods
.changes/next-release/cli-input-stdin.json	Changelog entry