fix: add PHPStan level 8 type guards to lib/functions.php (Cacti#6757)

* fix: add PHPStan level 8 type guards to lib/functions.php

Add defensive type checks (is_array, !== false, int casts) to resolve
PHPStan level 8 errors. No behavioral changes for normal operation;
only error paths (db failure, file open failure) are affected.

Signed-off-by: Thomas Vincent <thomasvincent@gmail.com>

* fix: update version_to_decimal @return docblock to int|string

Signed-off-by: Thomas Vincent <thomasvincent@gmail.com>

* fix: remove redundant is_array() checks and fix PHPStan errors in functions.php

- remove redundant is_array() guards paired with cacti_sizeof() (same variable)
- assign temp variable before prepare_validate_result() calls (by-ref param)
- remove dead is_array() guard on array_rekey() return (always array)

Signed-off-by: Thomas Vincent <thomasvincent@gmail.com>

* fix: strict === comparison and return type for sanitize_unserialize_selected_graphs

Signed-off-by: Thomas Vincent <thomasvincent@gmail.com>

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Signed-off-by: Thomas Vincent <thomasvincent@gmail.com>
Co-authored-by: TheWitness <thewitness@cacti.net>

Author: somethingwithproof