Fix result.to_dict() following new 2.0 typing

[valente]

Following on the changes in #13793, I think we should also stop trying to serialize into a dictionary since the header should already be a dictionary.

Currently we face the following issue when doing result.to_dict()

  File "/opt/app-root/lib64/python3.11/site-packages/qiskit/result/result.py", line 91, in to_dict
    "header": None if self.header is None else self.header.to_dict(),
                                               ^^^^^^^^^^^^^^^^^^^
AttributeError: 'dict' object has no attribute 'to_dict'

Added corresponding regression test based on existing to_repr test

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[ElePT]

Thanks for the fix! This must have been an oversight from the qobj removal. To make sure it doesn't slip into the cracks again, could you please add a unit test for the fix? We'd also need a release note.

[coveralls]

Pull Request Test Coverage Report for Build 14167128266

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 114 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.02%) to 88.082%

---

Files with Coverage Reduction	New Missed Lines	%
crates/accelerate/src/unitary_synthesis.rs	1	94.79%
crates/qasm2/src/lex.rs	3	92.48%
crates/cext/src/exit_codes.rs	7	0.0%
crates/cext/src/sparse_observable.rs	103	0.0%

Totals
Change from base Build 14129997569:	0.02%
Covered Lines:	72754
Relevant Lines:	82598

---

💛 - Coveralls

[ElePT]

Thanks a lot again @valente for catching this oversight. I pushed a few changes directly to the branch because of time pressure, as the release is planned for today and this fix unblocks the migration of internal packages to 2.0. I realized that we actually didn't need the release note, as the original PR is technically not released. I also added explicit typing for the remaining input arguments of Result to try to avoid future ambiguities. I am approving but we cannot merge the PR until the CLA is signed. Would you mind clicking on the CLA comment above to get it signed asap?