fix: SamConfig.get_all() mutates shared document, leaking one command's params into another's

[Adityaj0]

Which issue(s) does this change fix?

Fixes #9181

Why is this change necessary?

SamConfig.get_all() merged the current command's section-specific parameters into the [global] section using a live reference into self.document rather than a copy:

global_params = config_content.get(DEFAULT_GLOBAL_CMDNAME, {}).get(section, {})
global_params.update(params.copy())
params = global_params.copy()

global_params here is the actual dict stored inside self.document["<env>"]["global"][section]. Calling .update() on it permanently writes the current command's values into the shared global section. Since self.document is cached across calls (_read() only re-reads from disk when self.document is falsy), this corruption persists in memory for the lifetime of the SamConfig instance.

The result: if a SamConfig object is reused across multiple get_all() calls for different commands (a valid, documented use of the public API — get_all() takes cmd_names per call for exactly this purpose), a later command silently inherits an earlier command's parameter values instead of the true global default.

How does this change work?

Copy the global-section dict before merging into it, instead of mutating the dict stored in self.document:

global_params = dict(config_content.get(DEFAULT_GLOBAL_CMDNAME, {}).get(section, {}))
global_params.update(params)
params = global_params

What tests ran and what were the results?

Added a regression test, test_get_all_does_not_leak_command_params_into_global_across_calls, in tests/unit/lib/samconfig/test_samconfig.py. It fails against the pre-fix code (reproducing the exact leak: build's resolved config wrongly includes deploy's stack_name/region) and passes after the fix.

Full unit suite (tests/unit) passes: 9387 passed, 25 skipped.
mypy on the changed file: clean.

Checklist

• Add/update tests for this change
• make pr passes locally (unit tests + mypy on changed files; full local make pr target run via pytest tests/unit -q and scoped mypy)
• Write a clear PR title and description

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