chore: update changelog for v3.15.9

[Conor-FOSSA]

Overview

Prep for release v3.15.9, which picks up this Ficus PR: https://github.com/fossas/ficus/pull/144 and these themis PRs: https://github.com/fossas/themis/pull/152, https://github.com/fossas/themis/pull/153.

Acceptance criteria

• Changelog is correct
• We pick up the latest release of Ficus
• We pick up the latest release of themis

Testing plan

Risks

Metrics

References

Checklist

• I added tests for this PR's change (or explained in the PR description why tests don't make sense).
• If this PR introduced a user-visible change, I added documentation into docs/.
• If this PR added docs, I added links as appropriate to the user manual's ToC in docs/README.ms and gave consideration to how discoverable or not my documentation is.
• If this change is externally visible, I updated Changelog.md. If this PR did not mark a release, I added my changes into an ## Unreleased section at the top.
• If I made changes to .fossa.yml or fossa-deps.{json.yml}, I updated docs/references/files/*.schema.json AND I have updated example files used by fossa init command. You may also need to update these if you have added/removed new dependency type (e.g. pip) or analysis target type (e.g. poetry).
• If I made changes to a subcommand's options, I updated docs/references/subcommands/<subcommand>.md.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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Walkthrough

The changelog adds a new release section for version 3.15.9 documenting two updates: enhanced licensing detection to identify additional proprietary licenses, and snippet scanning updated to support TLS operation using the platform's native certificate store. No changes were made to release notes for version 3.15.8 or earlier.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and accurately describes the main change: updating the changelog for version 3.15.9, which aligns with the changeset.
Description check	✅ Passed	The description covers the overview with clear context and acceptance criteria, but testing plan, risks, and metrics sections are empty. The critical sections for a changelog PR are present.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[nficca]

Approving but I don't think we ever reference private projects in this changelog; suggested an alternative.

[nficca]

Since ficus and themis are private repos, I don't think it's very useful to link them here. Perhaps instead we could do something like:

Suggested change

	- Licensing: Update themis dependency to include LICENSE.spdx file parsing ([fossas/themis#152](https://github.com/fossas/themis/pull/152), [fossas/themis#153](https://github.com/fossas/themis/pull/153))
	- Snippet scanning: Update ficus dependency to improve snippet scan performance ([fossas/ficus#144](https://github.com/fossas/ficus/pull/144))
	- Improvements for licensing and snippet scanning ([#1649](https://github.com/fossas/fossa-cli/pull/1649))
	- Licensing: Additional proprietary license detection
	- Snippet scanning: Support TLS operation with platform native certificate store