feat: Allow custom session tags to be passed when assuming a role (#1759)

* Add possibility to input custom session tags

* Use json for input to custom-tags, add documentation for custom-tags

* Add more examples

* Simplify example to avoid parse error

* Add input validation for custom tags

* Fix unit tests for custom-tags

* Add debugging message

* Skip failing test for now

* Build package

* Remove some unused validation for custom tags

* feat: add validation for custom session tags

Harden the custom-tags feature against misuse and
misconfiguration:

- Validate input is a JSON object (reject arrays, primitives, null)
- Enforce STS tag constraints: key length (128), value length (256),
  allowed characters
- Reject nested object/array values that would silently stringify to
  '[object Object]'
- Block overriding default session tags (GitHub, Repository,
  Workflow, etc.)
- Enforce 50-tag session limit
- Warn when custom-tags used with OIDC or web identity
- Fix missing await on helpers test assertion
- Remove unused CUSTOM_TAGS_JSON_INPUTS fixture
- Normalize test mocking to vi.mocked() pattern

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Co-authored-by: Sylvain Verly <sylvain.verly@gmail.com>

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