Add a payment_metadata map in blinded * path contexts

[TheBlueMatt]

Similar to how BOLT 11 payments can use a `payment_metadata` to
provide arbitrary bytes in the invoice to be communicated back to
them when receiving, its useful to be able to provide some bytes
which are communicated back upon receiving a payment.

Here we do so in the BOLT 12 blinded * contexts, offering a
`BTreeMap<u64, Vec<u8>>` instead to enable more easily including
multiple sets of data.

Also note that a `Router` building a blinded path is allowed to
modify the `payment_metadata` without breaking the payment.

Tests by claude

We do so both in the blinded message and payment paths, supporting async payments when data is injected in the blinded payment paths (eg via the Router). We don't expose building offers with metadata yet.

[ldk-reviews-bot]

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[ldk-claude-review-bot]

No issues found.

All previously flagged issues from prior review passes have been verified as resolved:

• "tighly" typo: fixed (no occurrences remain)
• OffersContext::StaticInvoiceRequested::payment_metadata doc reference concern: was a misread; the doc at messenger.rs:474 correctly references OffersContext::InvoiceRequest::payment_metadata which exists

Verification completed across all changed files covering:

• TLV serialization: Odd type numbers (1) for all new payment_metadata fields ensure backwards compatibility. Ascending TLV order maintained in all structs/enums.
• Macro (option, encoding) pattern: The .as_ref().map(|f| $encoding(f)) expansion in _encode_tlv! (line 84) and _get_varint_length_prefixed_tlv_length! (line 256) both produce BigSizeKeyedMap<&BTreeMap<...>> which has a matching Writeable impl. Reference-type impls for AccountableBool<&bool>, HighZeroBytesDroppedBigSize<&T>, WithoutLength<&&String>, and WithoutLength<&&Features> correctly support the macro expansion.
• BigSizeKeyedMap DoS safety: Deserialization uses LengthReadable with LengthLimitedRead, bounding total read bytes by TLV length. No pre-allocation based on declared count. Duplicate keys rejected.
• Metadata propagation: Flows correctly through all paths — offer message path context → channelmanager extraction → invoice builder → payment context → blinded payment path → Event::PaymentClaimable. The AsyncBolt12OfferContext → Bolt12OfferContext conversion in channelmanager.rs properly carries payment_metadata.
• Pattern match exhaustiveness: All 9 pattern matches on OffersContext::InvoiceRequest across 6 files correctly handle the new payment_metadata field.
• Test coverage: New tests router_modifies_payment_metadata_in_blinded_path, pays_for_offer_with_payment_metadata_in_invoice_request_context, and async_payment_delivers_payment_metadata cover the three main metadata injection paths.

[codecov]

Codecov Report

❌ Patch coverage is 85.26316% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.15%. Comparing base (42e198c) to head (b8375a7).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/util/ser.rs	74.07%	1 Missing and 6 partials ⚠️
lightning/src/blinded_path/payment.rs	71.42%	6 Missing ⚠️
lightning/src/util/test_utils.rs	91.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4584   +/-   ##
=======================================
  Coverage   87.15%   87.15%           
=======================================
  Files         161      161           
  Lines      109251   109338   +87     
  Branches   109251   109338   +87     
=======================================
+ Hits        95215    95296   +81     
+ Misses      11560    11559    -1     
- Partials     2476     2483    +7     

Flag	Coverage Δ
fuzzing-fake-hashes	31.19% <16.25%> (+0.03%)	⬆️
fuzzing-real-hashes	22.91% <1.25%> (-0.02%)	⬇️
tests	86.22% <85.26%> (+<0.01%)	⬆️

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[ldk-reviews-bot]

🔔 1st Reminder

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