Fix transaction creation input validation and fee rate calculation

[tvpeter]

Description

This PR addresses input-validation problems on the transaction-building commands (create_tx, create_sp_tx, bump_fee), transaction fee rate and OP_RETURN data size:

• Panics on malformed input: create_tx and bump_fee called .unwrap() on Results so they panic (exit 101) instead of an error (exit 1).
• The create_sp_tx already guarded these paths, but improvements were made to the error type been returned
• Fee rates: --fee_rate was an f32 cast with as u64, which truncates and saturates, and a None from from_sat_per_vb was silently skipped. Parsing now happens in a value_parser, so bad values are rejected with
  a usage message before a wallet is loaded. Because FeeRate counts sat/kwu, fractional rates keep 1/250 sat/vB precision instead of being truncated .
• OP_RETURN size: Both --add_data and --add_string document "max 80 bytes" and neither enforced it. This has now been updated to 100_000 bytes and enforced in transaction building.
• create_dns_tx was had the same fee-rate bug and the same OP_RETURN handling, and has been fixed too.
• bump_fee --utxos and send_payjoin -f are fixed by the same changes.

Fixes #325

Notes to the reviewers

Changelog notice

• Fixed create_tx and bump_fee panicking on malformed --utxos and --add_data values instead of returning an error
• Fixed --fee_rate silently truncating to a whole sat/vB, falling back to a default, or producing a zero-fee transaction; unusable values are now rejected
• Enforced the documented 100_000 byte limit on --add_data and --add_string OP_RETURN payloads

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[codecov]

Codecov Report

❌ Patch coverage is 86.41975% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.75%. Comparing base (5afbc8a) to head (4a69235).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/handlers/offline.rs	50.00%	6 Missing ⚠️
src/handlers/dns/mod.rs	20.00%	4 Missing ⚠️
src/handlers/payjoin/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #326      +/-   ##
==========================================
+ Coverage   57.78%   58.75%   +0.97%     
==========================================
  Files          22       22              
  Lines        3733     3773      +40     
==========================================
+ Hits         2157     2217      +60     
+ Misses       1576     1556      -20     

Flag	Coverage Δ
rust	58.75% <86.41%> (+0.97%)	⬆️

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