feature: add weighted PauliString sums

[caijintaoduolacmeng-star]

Issue #, if available:
Closes #1256

Description of changes:

• Adds a PauliStringSum container for weighted sums of PauliString objects.
• Supports addition, subtraction, scalar multiplication, multiplication by PauliString, iteration, lookup, and coefficient normalization.
• Exports PauliStringSum from braket.quantum_information.

Testing done:

• ..venv\Scripts\python -m pytest -o addopts='' test\unit_tests\braket\quantum_information\test_pauli_string.py -q
• ..venv\Scripts\python -m ruff check src\braket\quantum_information\pauli_string.py

Merge Checklist

General

• I have read the CONTRIBUTING doc
• I used the PR title format described in CONTRIBUTING
• I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have checked that my tests are not configured for a specific region or account (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

❌ Patch coverage is 78.63248% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.77%. Comparing base (a5ee47d) to head (1a37e7e).

Files with missing lines	Patch %	Lines
src/braket/quantum_information/pauli_string.py	78.44%	18 Missing and 7 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #1282      +/-   ##
===========================================
- Coverage   100.00%   99.77%   -0.23%     
===========================================
  Files          169      169              
  Lines        10963    11078     +115     
  Branches      1412     1434      +22     
===========================================
+ Hits         10963    11053      +90     
- Misses           0       18      +18     
- Partials         0        7       +7     

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