refactor(secret-resolver)!: refactor to use constants for base path and env var prefix

[cassiofariasmachado]

Description

Reorders read_from_mount_and_fallback_to_env_var parameters so module, instance, and target come first, and makes base_volume_mount/base_var_name optional kwargs with constant defaults.

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring

How to Test

1. Run the unit tests: uv run pytest -m "not integration" -q
2. Verify callers no longer need to pass base_volume_mount or base_var_name:

   from sap_cloud_sdk.core.secret_resolver import read_from_mount_and_fallback_to_env_var
   read_from_mount_and_fallback_to_env_var(module="myservice", instance="default", target=cfg)

3. Verify overrides still work via keyword arguments:

   read_from_mount_and_fallback_to_env_var(
       "myservice", "default", cfg,
       base_volume_mount="/custom/path",
       base_var_name="MY_PREFIX",
   )

Checklist

• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have updated documentation (if applicable)
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages

Breaking Changes

• read_from_mount_and_fallback_to_env_var parameter reorder: The first two positional parameters changed from (base_volume_mount, base_var_name, module, instance, target) to (module, instance, target, base_volume_mount=..., base_var_name=...). Any caller that used positional arguments must be updated.

  Migration: Update call sites to the new positional order, or switch to keyword arguments:

  # Before
  read_from_mount_and_fallback_to_env_var("/etc/secrets/appfnd", "CLOUD_SDK_CFG", "mymod", "inst", cfg)
  
  # After (keyword args — most explicit)
  read_from_mount_and_fallback_to_env_var(module="mymod", instance="inst", target=cfg)
  
  # After (custom path via kwargs)
  read_from_mount_and_fallback_to_env_var("mymod", "inst", cfg, base_volume_mount="/custom", base_var_name="MY_PREFIX")